Spanish or Castilian (Spanish: español or castellano) is an Iberian Romance language, and the fourth most widely spoken language in the world according to some sources, while other sources list it as the 2nd or 3rd most spoken language. It is spoken as a first language by about 352 million people, or by 417 million including non-native speakers (according to 1999 estimates). Some assert that, after English, Spanish can now be considered the second most important language in the world (probably replacing even French), due to its increased usage in the United States, the high birth rate in most of the countries where it is official, the growing economies of the Spanish-speaking world, its enormous influence on the global music market, and simply due to the broad number of areas on the Earth's surface that the language is spoken in.

"Spanish" or "Castilian"
Spaniards tend to call this language español (Spanish) when contrasting it with languages of other states (for example: in a list with French and English), but call it castellano (Castilian, from the Castile region) when contrasting it with other languages of Spain (such as Galician, Basque, and Catalan). For the rest of the Spanish-speaking world, speakers of the language in some areas refer to it as español, and in others castellano is more common. Castellano is the name given to Spanish language in Argentina, Bolivia, Chile, Ecuador, Paraguay, Perú, Uruguay and Venezuela.

Some philologists use Castilian only when speaking of the language spoken in Castile during the Middle Ages, stating that it is preferable to use Spanish for its modern form. Castilian can be also a subdialect of Spanish spoken in most parts of modern day Castile. It would have a series of characteristics and a specific pronunciation different to the one of Andalusia or Aragon for example, where they would speak different subdialects.

Classification and related languages
Spanish is a member of the Romance branch of Indo-European, descended largely from Latin and having much in common with its European geographical neighbors.

Spanish is related to several languages in terms of phonology, grammar and orthography. Of these, Portuguese is perhaps one of the most similar in terms of major languages. However, Spanish is also closely related to Catalan, Asturian, Galician and several other Romance languages. Spanish has fewer similarities with French and Italian but shares strong ties due to Latin roots.

Portuguese is orthographically similar in many ways to Spanish but it has a very distinctive phonology. A speaker of one of these languages may require some practice to effectively understand a speaker of the other (although generally it is easier for a Portuguese native speaker to understand Spanish than the other way around). Compare, for example:

Ela fecha sempre a janela antes de jantar. (Portuguese)
Ella cierra siempre la ventana antes de cenar. (Spanish)
Some less common phrasings and word choices have closer cognates in Spanish because Portuguese has managed to retain a much larger vocabulary, with stronger Latin heritage:

Ela cerra sempre a janela antes de cear. (less common Portuguese)
(Which translates as "She always closes the window before having dinner").

In some places, Spanish and Portuguese are spoken almost interchangeably. Portuguese speakers are generally able to read Spanish, and Spanish speakers are generally able to read Portuguese, even if they cannot understand the spoken language. In fact, the number of bilingual speakers in Brazil (where Portuguese is the official language) has greatly risen because nearly every nation bordering Brazil is Spanish speaking.

History
The Spanish language developed from vulgar Latin, with influence from Celtiberian, Basque and Arabic, in the north of the Iberian Peninsula (see Iberian Romance languages). Typical features of Spanish diachronical phonology include lenition (Latin vita, Spanish vida), palatalization (Latin annum, Spanish año) and diphthongation (stem-changing) of breve E/O from vulgar Latin (Latin terra, Spanish tierra; Latin novus, Spanish nuevo). Similar phenomena can be found in most other Romance languages as well.

During the Reconquista, this northern dialect was carried south, and indeed is still a minority language in northern Morocco.

The first Latin to Spanish dictionary (Gramática de la Lengua Castellana) was written in Salamanca, Spain in 1492 by Elio Antonio de Nebrija. When Isabella of Castile was presented with the book, she asked, What do I want a work like this for, if I already know the language?, to which he replied, Ma'am, the language is the instrument of the Empire.

From the 16th century on, the language was brought to the Americas, Federated States of Micronesia, Guam, Marianas, Palau and the Philippines by Spanish colonization.

In the 20th century, Spanish was introduced in Equatorial Guinea and Western Sahara.

For details on borrowed words and other external influences in Spanish, see Influences on the Spanish language.

Geographic distribution
Spanish is one of the official languages of the United Nations and the European Union. The majority of its speakers are confined to the Western Hemisphere, Europe and the Spanish territories in Africa (Canary Islands, Ceuta and Melilla).

With approximately 106 million first-language and second-language speakers, Mexico boasts the largest population of Spanish-speakers in the world. The four next largest populations reside in Colombia (44 million), Spain (c. 44 million), Argentina (39 million) and the United States of America (U.S. residents age 5 and older who speak Spanish at home number 31 million)

Spanish is the official and most important language in 20 countries: Argentina, Bolivia (co-official Quechua and Aymara), Chile, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, El Salvador, Equatorial Guinea (co-official French), Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay (co-official Guaraní), Peru (co-official Quechua and Aymara), Puerto Rico (co-official English), Spain (co-official Catalan/Valencian, Galician and Basque), Uruguay, Venezuela, and Western Sahara (co-official Arabic).

In Belize, Spanish holds no official recognition, however, it is the native tongue of about 50% of the population, and is spoken as a second language by another 20%. It is arguably the most important and widely-spoken on a popular level, but English remains the sole official language.

In the United States, Spanish is spoken by three-quarters of its 41.3 million Hispanic population. It is also being learned and spoken by a small, though slowly growing, proportion of its non-Hispanic population for its increasing use in business, commerce, and both domestic and international politics. Spanish does hold co-official status in the state of New Mexico, and in the unincorporated U.S. territory of Puerto Rico. See Spanish in the United States for further information.

In Brazil, Spanish has obtained an important status as a second language among young students and many skilled professionals. In recent years, with Brazil decreasing its reliance on trade with the USA and Europe and increasing trade and ties with its Spanish-speaking neighbours (especially as a member of the Mercosur trading bloc), much stress has been placed on bilingualism and Spanish proficiency in the country. On July 07, 2005, the National Congress of Brazil gave final approval to a bill that makes Spanish a second language in the country’s public and private primary schools. The close genetic relationship between the two languages, along with the fact that Spanish is the dominant and official language of almost every country that borders Brazil, adds to the popularity. Standard Spanish and Ladino (Judæo-Spanish spoken by Sephardic Jews) may also be spoken natively by some Spanish-descended Brazilians, immigrant workers from neighbouring Spanish-speaking countries and Brazilian Sephardim respectively, who have maintained it as their home language. Additionally, in Brazil's border states that have authority over their educational systems, Spanish has been taught for years. In many other border towns and villages (especially along the Uruguayo-Brazilian border) a mixed language commonly known as Portuñol is also spoken.

In European countries other than Spain and Andorra (where it holds no official status), it may be spoken by some of their Spanish-speaking immigrant communities, primarily in the Netherlands, Italy, France, Germany and the United Kingdom where there is a strong community in London. There has been a sharp increase in the popularity of Spanish in the UK over the last few years. It is spoken by much of the population of Gibraltar, though English remains the only official language. Yanito, an English-Spanish mixed language is also spoken.

Among the countries and territories in Oceania, Spanish is the seventh most spoken language in Australia. It is also spoken by the approximately 3,000 inhabitants of Easter Island, a territorial possession of Chile. The island nations of Guam, Palau, Northern Marianas, Marshall Islands and Federated States of Micronesia all once had Spanish speakers, but Spanish has long since been forgotten, and now only exists as an influence on the local native languages.

In Asia the Spanish language has long been in decline. Spanish ceased to be an official language of the Philippines in 1987, and it is now spoken by less than 0.01% of the population; 2,658 speakers (1990 Census). However, the sole existing Spanish-Asiatic creole language, Chabacano, is also spoken by an additional 0.4% of the Filipino population; 292,630 (1990 census). Most other Philippine languages contain generous quantities of Spanish loan words. Among other Asian countries, Spanish may also be spoken by pockets of ex-immigrant communities, such as Mexican-born ethnic Chinese deported to China or third and fourth generation ethnic Japanese Peruvians returning to their ancestral homeland of Japan.

Spanish is also spoken by segments of the populations in Aruba, Canada, Curaçao, Israel (both standard Spanish and Ladino), northern Morocco (both standard Spanish and Ladino), Netherlands Antilles, Trinidad and Tobago, Turkey (Ladino), and the U.S. Virgin Islands.

In the Antarctic, the territorial claims and permanent bases made by Argentina, Chile, Peru and Spain also place Spanish as the official and working language of these enclaves.

Variations
There are important variations among the various regions of Spain and Spanish-speaking America. In Spain the North Castilian dialect pronunciation is commonly taken as the national standard (although the characteristic weak pronouns usage or laísmo of this dialect is deprecated).

Spanish has three second-person singular pronouns: tú, usted, and in some parts of Latin America, vos (the use of this form is called voseo). Generally speaking, tú and vos are informal and used with friends (though in Spain "vos" is considered a highly exalted archaism that is now confined to liturgy). Usted is universally regarded as the formal form, and is used as a mark of respect, as when addressing one's elders or strangers. The pronoun "vosotros" is the plural form of "tú" in most of Spain, although in the Americas (and some particular southern-Spain cities such as Cádiz) it is replaced with "ustedes." It is remarkable that the informal use of "ustedes" in southern Spain does not keep the proper pronoun-verb relationship: while the formal form of "you go" would be "ustedes van", in Cádiz the informal form would be constructed as "ustedes váis", making use of the second person of the plural instead of the third (which constitutes the formal construction).

Vos is used extensively as the primary spoken form of the second-person singular pronoun in various countries around Latin America, including Argentina, Costa Rica, Ecuador, El Salvador, Guatemala, Honduras, Nicaragua, Paraguay and Uruguay. In Argentina, Uruguay, and increasingly in Paraguay, is it also the standard form used in the media, whereas media in other voseante countries continue to use usted or tú. Vos may also be present in other countries as a limited regionalism. Its use, depending on country and region, can be considered the accepted standard or reproached as sub-standard and considered as speech of the ignorant and uneducated. The interpersonal situations in which the employment of vos is acceptable may also differ considerably between regions.

Spanish forms also differ regarding second-person plural pronouns. The Spanish dialects of Latin America have only one form of the second-person plural; ustedes (formal/familiar). Meanwhile, in Spain there are two; ustedes (formal) and vosotros (familiar/informal).

The RAE (Real Academia Española), in association with twenty-one other national language academies, exercises a controlling influence through its publication of dictionaries and widely respected grammar guides and style guides. In part due to this influence, and also because of other socio-historical reasons, a neutral standardized form of the language (Standard Spanish) is widely acknowledged for use in literature, academic contexts and the media.