Spanish or Castilian (castellano) is an Indo-European, Romance language that originated in northern Spain, and gradually spread in the Kingdom of Castile and evolved into the principal language of government and trade. It was taken to Africa, the Americas, and Asia Pacific with the expansion of the Spanish Empire between the fifteenth and nineteenth centuries. Today, between 322 and 400 million people speak Spanish as a native language, making it the world's second or third most-spoken language by native speakers, depending on the sources.

Spanish is one of the official languages of the European Union, the Organization of American States, the Organization of Ibero-American States, the United Nations, and the Union of South American Nations. It is estimated that the combined total of native and non-native Spanish speakers is approximately 500 million, likely making it the third most spoken language by total number of speakers (after English and Chinese). Today, Spanish is an official language of Spain, most Latin American countries, and Equatorial Guinea; 20 nations speak it as their primary language. Spanish also is one of six official languages of the United Nations. Mexico has the world's largest Spanish-speaking population, and Spanish is the second most-widely spoken language in the United States and the most popular studied foreign language in U.S. schools and universities. Global internet usage statistics for 2007 show Spanish as the third most commonly used language on the Internet, after English and Chinese.

In the 2006 census, 44.3 million people of the U.S. population were Hispanic or Latino by origin; 34 million people, 12.2 percent, of the population older than 5 years speak Spanish at home. Spanish has a long history in the United States (many south-western states were part of Mexico and Spain), and it recently has been revitalized by much illegal immigration from Latin America. Spanish is the most widely taught foreign language in the country. Although the United States has no formally designated "official languages," Spanish is formally recognized at the state level in various states besides English; in the U.S. state of New Mexico for instance, 30% of the population speaks the language. It also has strong influence in metropolitan areas such as Los Angeles, Miami, San Antonio, New York City, and in the 2000s the language has rapidly expanded in Atlanta, Houston, Phoenix and other major Sun-Belt cities. Spanish is the dominant spoken language in Puerto Rico, a U.S. territory. In total, the U.S. has the world's fifth-largest Spanish-speaking population.

There are important variations among the regions of Spain and throughout Spanish-speaking America. In some countries in Hispanophone America, it is preferable to use the word castellano to distinguish their version of the language from that of Spain, thus asserting their autonomy and national identity. In Spain, the Castilian dialect's pronunciation is commonly regarded as the national standard, although a use of slightly different pronouns called laísmo of this dialect is deprecated. More accurately, for nearly everyone in Spain, "standard Spanish" means, "pronouncing everything exactly as it is written," an ideal which does not correspond to any real dialect, though the northern dialects are the closest to it. In practice, the standard way of speaking Spanish in the media is "written Spanish" for formal speech, "Madrid dialect" (one of the transitional variants between Castilian and Andalusian) for informal speech.

Spanish evolved from Vulgar Latin, with major influences from Arabic in vocabulary during the Andalusian period and minor surviving influences from Basque and Celtiberian, as well as Germanic languages via the Visigoths. Spanish developed along the remote cross road strips among the Alava, Cantabria, Burgos, Soria and La Rioja provinces of Northern Spain (see Glosas Emilianenses), as a strongly innovative and differing variant from its nearest cousin, Leonese, with a higher degree of Basque influence in these regions (see Iberian Romance languages). Typical features of Spanish diachronical phonology include lenition (Latin vita, Spanish vida), palatalization (Latin annum, Spanish año, and Latin anellum, Spanish anillo) and diphthongation (stem-changing) of short e and o from Vulgar Latin (Latin terra, Spanish tierra; Latin novus, Spanish nuevo). Similar phenomena can be found in other Romance languages as well. During the Reconquista, this northern dialect from Cantabria was carried south, and remains a minority language in the northern coastal Morocco. The first Latin-to-Spanish grammar (Gramática de la Lengua Castellana) was written in Salamanca, Spain, in 1492, by Elio Antonio de Nebrija. When it was presented to Isabel de Castilla, she asked, "What do I want a work like this for, if I already know the language?", to which he replied, "Your highness, the language is the instrument of the Empire."

From the 16th century onwards, the language was taken to the Americas and the Spanish East Indies via Spanish colonization. In the 20th century, Spanish was introduced to Equatorial Guinea and the Western Sahara, and in areas of the United States that had not been part of the Spanish Empire, such as in Spanish Harlem, in New York City. For details on borrowed words and other external influences upon Spanish, see Influences on the Spanish language.

Spanish is closely related to the other West Iberian Romance languages: Asturian, Galician, Ladino, Leonese and Portuguese. Catalan, an East Iberian language which exhibits many Gallo-Romance traits, is more similar to the neighboring Occitan language than to Spanish, or indeed than Spanish and Portuguese are to each other. Spanish and Portuguese share similar grammars and vocabulary as well as a common history of Arabic influence while a great part of the peninsula was under Islamic rule (both languages expanded over Islamic territories). Their lexical similarity has been estimated as 89%.

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