The West Slavonians, who settled down in the area of present day Slovakia, separated by language from Proto-Slavic in the 10th century and their language further developed within the different dialect groups. An important step was the connection of the Nitra principality to the Moravian principality and the formation of the Great Moravian Empire in 833. A particularly important chapter in the development of the Slavic language in that period and territory is connected with the mission of the apostles Cyril (Constantine) and Methodius from Thessaloniki, who were sent by the Emperor Michael III at the request of the Great Moravian Prince Rastislav in 863 to the Great Moravia to disseminate Christianity and who laid the foundation of the legal order in a Slavic language. For this purpose Constantine developed a special alphabet called the Glagolitic alphabet, based on the Greek minuscule, and used it for the translation of the Gospel and the Byzantine book of laws to Old Church Slavonic. The brothers obtained permission for use of the new letters as a liturgical language ("lingua quarta") from Pope Hadrianus II.
Following the integration of the territory of present day Slovakia to the early feudal Kingdom of Hungary, Slovak dialects and the roots of Slovak nationality began developing in the 10th to 14th centuries. In particular, Latin was used as an administrative language as well as a liturgical language in the West Church, and as the language of state administration it survived until the middle of the 19th century. From the beginning, local languages were only used for preaching and other pastoral acts. In the 15th century, due to Hussite expeditions, the Brethren movements, and students of Czech universities, the Czech language started to assert itself in Slovakia, where the language was spoken with Slovakisms. The first evidence of this linguistic situation and the emerging Slovak language is found in the Book of Žilina (1473), which contains the text of the Magdeburg law written in Slovakized Czech. From the 15th century, within the territory of the present day Slovakia, Czech, in addition to Latin, gradually assumed the role of a literary language used by the educated. In the 16th and 17th centuries, it also became a liturgical language for Slovak Protestants; it played an important role in Protestant documents until the 19th century, and in another form, until the end of the 20th century. From the beginning, Slovaks used Czech only in written form, and never perceived it as their own language. By the end of the 18th century, in the cities of western Slovakia and later on in central Slovakia, domestic literary dialects began to emerge and were used by the burghers and the intelligentsia. Cultural West Slovak and cultural Central Slovak became the basis for the standard language codifications by Anton Bernolák in the end of the 18th century and by Ľudovít Štúr in the 1840's. Basing his codification on the urban languages of several central Slovakian cities, in 1846, Ľudovít Štúr issued the normative grammar "Nauka reči Slovenskej" (Scientific Slovak Language), which codified the Slovak standard language. This standard, with some modifications, is the basis for today's Slovak language.
After the formation of the Czechoslovak Republic, peace treaties and the Constitution of 1920 defined a common "Czechoslovak nation" and "Czechoslovak language" in two versions (Czech and Slovak). The spelling of Slovak was confirmed by the Rules of Slovak Orthography in 1931. A different set of rules, the "Matica usage" was published by the Matica Slovenská Institute. This institute had purist tendencies, which it strongly promoted in the magazine"Slovenská reč", which was published beginning in 1932. In Slovak, the purism manifested itself with a delay against neighbouring countries – especially in the interwar period. However, Slovak remained an "open language", able to assimilate new words from domestic sources, as well as enriching itself through borrowings from other languages. After World War II, the Slovak language developed to a form in which it could fulfil all of the functions and needs of modern society. In this period, it spent several turbulent years evolving, but little by little, it was described and codified at all levels.
The situation in Czechoslovakia (Czecho-Slovakia) was characterised by bilingualism and so-called semi-communication. Czechs and Slovaks used their own languages to communicate with one another, but they understood each other quite well, and continue to understand each other today, though they are now independent states. They are still able to make themselves understood without serious problems, although younger generations that were linguistically socialized only after the division of the former Czechoslovakia (Czecho-Slovakia) may have certain problems with communication. TV broadcasting also helps maintain a high level of mutual comprehension. In general, however, Slovaks are able to understand the Czech language more easily than Czechs can understand Slovak.
The year 1989 marked a new milestone in the development of the Slovak language. Many archaisms and words closely related to the socialist economy and system of government disappeared, and many Anglicisms and internationalisms were absorbed into the present language. However, these new additions are no risk to the existence of Slovak. Throughout its history, the Slovak language has always been open to loanwords, so new loanwords present no threat. It adopted lexical elements from other languages with which it came into contact, especially from Latin (and Greek, by way of Latin), Czech, German, French, Italian, and to a lesser degree, from Hungarian. More recently, loanwords have been adopted from English, especially from American English.
On 1 May 2004, Slovak became one of the official languages of the European Union.