The issue is a bit convoluted to say it lightly. The English term slave (also German Sklave, French esclave etc.) are cognate with the word Slav. It is derived from the Latin word sclavus, but this generally is not Ancient Latin, but rather one used in later part of the Early Middle Ages (Ancient Latin words for slave were usually servus or ancillus). This is because southern Slaves living in Illyria and surrounding lands were often raided and enslaved by the inhabitants of the Apeninian Peninsula. These people were known in Greek as Sklavenoi, Sklabinoi or Sklabenoi and were most like ancestors of Slovenians. But back then there was no single word denoting Slavs and possibly they were not considered a single supergroup as the are known today. Slavs living in the north (Western Slavs) were generally called Vends or Veneds (relic of this designation can be found in terms like 'Wendish Crusade' or 'Wendish Hanzeatic Group') in e.g. Tacitus' Germania, Pliny's Historia Naturalis or Ptolemy's Geografia. Eastern Slavs were referred to as Antes (although Ptolemy calles them Suobenoi and states that they are inhabiting areas around river Rha (interpreted as Volga). Southern Slavs were also known as the aforementioned Sklavinoi (it is also possible that Ptolemy conflated southern and eastern Slavs what makes sense given the common migration in the region). Also Iordanes, author of Getica who lived in 6th century, mentions the divisions between 'Veneds', 'Ants/Antes' and 'Sclaveni', noting that these large groups are further divided into particular nations of different names. It is most likely that due to the prevalence of Greek and Latin in the medieval diplomacy and scholarship, the name referring to only one subgroup of Slavs has been eventually applied to an entire ethnolinguistic group.

There is also a theory presented by Friedrich Kluge that the term Sklabenoi might have not necessarily been derived from a specific group but rather from the Middle Greek skyleúo, meaning 'taking trophies' that was mistakenly taken for the ethnonym of the Slavic Illyrians and given the aforementioned prevalence of Greek and Latin, eventually sort of stuck. In this theory, the modern term Slovenes would be a derivative of the Greek word mentioned above.

Due to their geographical location, Greeks had little contact with Western Slavs and limited contact with Eastern Slavic groups, but they could have quite easily contacted with southern Slavic people. It is thus likely that the term recorded by Procopius and others (Sklabenoi, Slabenoi etc.) might have referred to a particular group of the southern Slavs that most likely gave their name to modern Slovenia. This idea is to some extent corroborated by 6th century poem by Martin of Braga collected in Basicica, where Sclavus is used among the names of other peoples inhabiting Western and southern Europe (Alamani, Saxons, Thuringians, Pannonians, Alans, Ostrogoths, Francs, Burgundians etc.), but does not mention any Slavic groups living in Central and Eastern Europe. In addition, even though in the 1st or 2nd century CE Western Slavs are consistently called Veneds, Jonas of Bobbio in his Vita Columbani written around 643 uses Veneds as a synonym of Sclaves while the famous illustration in the Gospel Book of Otto III made around 1000 CE uses the term Sclavinia as the generalized 'Slavic lands' and Helmold's Chronica Sclavorum uses the term Slavs in a modern sense, as a joint designation of Western, Eastern and Southern Slavic people, with Veneds (also rendered as Vinuli) being identified as Polabian Slavs residing around the northern part of the Oder river.

Last but not least it is worth noting that the terms derived from Latin sclavus are present only in Western European languages that were influenced by Romance ones. In Slavic languages, the term 'slave; is generally cognate with words like 'unfree' (cf. Polish niewolnik) 'child' (cf. Czech and Slovak otrok) or 'work' (cf. Russian or Ukrainian rab). Even the old Germanic and Scandinavian languages used various terms like thrall or theow, that are still present in Icelandic thrael). Interestingly, in modern Greek, the word for slave is doulos.

Thus, it is possible that the term slave has been directly created from the Greek name of a certain south Slavic people or group of peoples and through the dominance of Greek and Latin literature in the expanding Christendom it became eventually applied to all the peoples speaking Slavic languages and inhabiting Central and Eastern Europe, displacing previously used ancient names of these ethnic groups.