Wikipedia
Thraco-Illyrian is a hypothesis that the Thraco-Dacian and Illyrian languages comprise a distinct branch of Indo-European. Thraco-Illyrian is also used as a term merely implying a Thracian-Illyrian interference, mixture or sprachbund, or as a shorthand way of saying that it is not determined whether a subject is to be considered as pertaining to Thracian or Illyrian. Downgraded to a geo-linguistic concept, these languages are referred to as Paleo-Balkan.
The linguistical hypothesis was especially current in the early 20th century, but after the 1960s it was seriously called into question. New publications argued that no strong evidence for Thraco-Illyrian exists, and that the two language-areas show more differences than correspondences (Vladimir Georgiev, Ivan Duridanov, Eric Hamp, et al.).