“Indo-European: Family of languages including, at historically its major limit, most of those spoken in Europe and, at its eastern limit, the major languages of all but the southern part of the Indian subcontinent. Usually divided into eleven main branches: in the order in which they are first attested, Anatolian (now extinct), Greek, Indo-Iranian, Italic (represented by the modern Romance languages), Celtic, Germanic (which includes English), Armenian, Tocharian (extinct), Slavic (Slavonic), Baltic (represented by Latvian and Lithuanian), and Albanian. Groupings larger than these are problematic to varying degrees: the safest hypothesis is that of a common Balt-Slavonic.
The comparative method has its origin in the intensive study of Indo-European, especially in the German-speaking universities, from the early 19th century. The size and complexity of the family, in comparison with many others that can be established with the same certainty, reflects in part the early date at which the forms in several branches can be compared.”
Excerpted from: Marshall, P.H., ed. The Oxford Concise Dictionary of Linguistics. New York: Oxford University Press, 2014.