Origins
The word 'Slavic' reached English through a chain of borrowings that begins with the Slavic peoples' own name for themselves. The Proto-Slavic self-designation was *Slověninъ (singular) or *Slověne (plural), first attested in the works of Byzantine Greek historians of the sixth century CE as 'Skláboi' (Σκλάβοι) or 'Sklavenoi' (Σκλαβηνοί). This was latinized as 'Sclaveni' or 'Sclavi' in Latin sources. The modern English adjective 'Slavic' (and its older variant 'Slavonic') derives from these medieval Latin forms.
The etymology of the native name is best understood as deriving from Proto-Slavic *slovo, meaning 'word' or 'speech.' Under this interpretation, the Slavs are literally 'the people of the word' — those who speak intelligibly, whose language one can understand. This interpretation is strengthened by a remarkable parallel: the Proto-Slavic word for the Germanic peoples was *němьci, derived from *němъ (mute, speechless, incomprehensible). The Slavs defined themselves as speakers and their neighbors as those who cannot speak — a classic us/them dichotomy based on linguistic comprehension. This naming survives in modern languages: Russian 'nemtsy' (немцы), Polish 'Niemcy,' Czech 'Němci,' and Hungarian 'német' all mean 'Germans' and all derive from the Slavic 'mute' root.
The PIE ancestor of *slovo is likely *ḱlew- (to hear), which produced an extraordinary family of words across the Indo-European languages: Greek 'kléos' (fame, glory — literally 'what is heard about someone'), Latin 'cluēre' (to be called, to have a reputation), and Sanskrit 'śrávas' (fame). English descendants through various routes include 'loud' (from Germanic) and 'Clio' (the muse of history, she who makes famous).
French Influence
The most infamous derivative of 'Slav' is 'slave.' During the early medieval period, large numbers of Slavic peoples were captured and sold in the slave markets of the Byzantine Empire, the Islamic world, and Western Europe. By the ninth and tenth centuries, Medieval Latin 'sclavus' had shifted from meaning 'a Slavic person' to meaning 'an enslaved person,' regardless of ethnicity. This sense passed into Old French 'esclave,' then into English 'slave' (first attested c. 1290). The semantic shift was complete: a people's name became a condition of bondage. This is one of the most consequential — and painful — etymological transformations in European history.
The Slavic language family is conventionally divided into three branches. East Slavic includes Russian, Ukrainian, and Belarusian. West Slavic includes Polish, Czech, Slovak, and Sorbian. South Slavic includes Bulgarian, Macedonian, Serbian, Croatian, Bosnian, Montenegrin, and Slovenian. All descend from a common ancestor, Proto-Slavic, which remained relatively unified until roughly the seventh century CE — remarkably late compared to the breakup of Proto-Germanic or Proto-Celtic.
The written history of Slavic begins with the mission of Saints Cyril and Methodius to Great Moravia in 863 CE. Cyril devised the Glagolitic alphabet (and his students later created the Cyrillic alphabet, named in his honor) to translate Christian scriptures into Old Church Slavonic, the first Slavic literary language. Old Church Slavonic, based primarily on the South Slavic dialect of ninth-century Thessaloniki, became the liturgical and literary language of Orthodox Slavic Christianity and holds a position in Slavic studies analogous to that of Latin in Romance studies.
Latin Roots
The adjective 'Slavic' was established as a linguistic term in the early nineteenth century, during the same period of comparative philology that formalized 'Germanic,' 'Romance,' and 'Celtic' as language-family labels. The older English form 'Slavonic' (modeled on 'Teutonic') was long preferred in British usage, while 'Slavic' prevailed in American and continental European scholarship. Today, 'Slavic' is the dominant form in linguistics, while 'Slavonic' survives mainly in the fixed expressions 'Old Church Slavonic' and 'Church Slavonic.'