[Verginius Rufus]

The term "Slav" comes from the Slavic languages. Slava in Russian means "glory" or "fame" (cognate with Ancient Greek kleos--the Slavic languages are "satem" languages which have an S where the "centum" languages like Latin, Greek, and the Germanic languages have a K [which became an H in the Germanic languages--so English "hundred" is cognate with Latin "centum"]).

Russian slovo means "word."

Among Slavic groups you get ethnic names or place names like Slovak/Slovakia, Slovene/Slovenia, Slavonia, etc. Either people thinking of themselves as the glorious ones or the ones who speak intelligibly (unlike the Vlachs and Germans and other non-Slavs).

The Latin term would be formed from the word the Slavs used for themselves.

[Rockingham]

The late Latin and Byzantine terms for Slavs, usually referring to Slavic people in bondage and traded in the late Middle Ages, were transmitted into English as “slave.”