Spanish or Castilian (español or castellano in Spanish) is a Romance language in the Ibero-Romance group that evolved from several dialects and languages in the northern fringes of the Iberian Peninsula during the 10th century and gradually spread through the Kingdom of Castile, becoming the foremost language for government and trade in the Spanish Empire.

Latin, the basis of all Romance languages including Spanish, was introduced to the Iberian Peninsula by the Romans during the Second Punic War around 210 BC. During the 5th century, Hispania was invaded by Germanic Vandals, Suevi and Visigoths, and other eastern peoples (Alans), but they left few linguistic influences other than a few dozen loanwords. After the Moorish Conquest in the 8th century, Arabic became a significant influence in the evolution of Iberian languages including Castilian (see Influences on the Spanish language).

Modern Spanish developed with the Readjustment of the Consonants (Reajuste de las sibilantes) that began in 15th century. The language continues to adopt foreign words from a variety of other languages, as well as developing new words. Spanish was taken most notably to the Americas as well as to Africa and Asia Pacific with the expansion of the Spanish Empire between the fifteenth and nineteenth centuries.

In 1999, there were 402 million people speaking Spanish as a native language and a total of 417 million people worldwide. Currently these figures up to 400 and 450 million people respectively. It is the second most natively spoken language in the world, after Mandarin Chinese. Mexico contains the largest population of Spanish speakers. Spanish is one of the six official languages of the United Nations.

Spanish evolved from Vulgar Latin introduced to the Iberian Peninsula by Romans during the Second Punic War around 210 BC, with influence from Arabic during the Andalusian period and other surviving influences from Basque and Celtiberian, as well as Germanic languages via the Visigoths.

Castilian is thought to have evolved in the northern fringes of the Iberian Peninsula during the 10th century along the remote crossroad strips among the Alava, Cantabria, Burgos, Soria and La Rioja provinces of Northern Spain , as a strongly innovative and differing variant from its nearest cousin, Leonese, with a higher degree of Basque influence in these regions .

Modern Spanish developed in Castile with the Readjustment of the Consonants (Reajuste de las sibilantes) during the 15th century. Typical features of Spanish diachronic phonology include lenition (Latin vita, Spanish vida), palatalisation (Latin annum, Spanish año, and Latin anellum, Spanish anillo) and diphthongisation (stem-changing) of stressed 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.

This northern dialect from Cantabria was carried south during the Reconquista.

The first Spanish grammar (Gramática de la lengua castellana) — and, incidentally, the first grammar of any modern European language — was written in Salamanca, Spain, in 1492, by Elio Antonio de Nebrija. When he presented it to Queen Isabella, according to anecdote, she asked him what was the use of such a work, and he answered that language is the instrument of empire. In his introduction to the grammar, dated August 18, 1492, Nebrija wrote that "... language was always the companion of empire."

From the 16th century onwards, the language was taken to the Americas and the Spanish East Indies via Spanish colonisation. Miguel de Cervantes Saavedra's influence on the Spanish language from the 17th century has been so great that Spanish is often called la lengua de Cervantes (the language of Cervantes).