from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Designating those Indo-European languages, including the Indo-Iranian, Armenian, and Balto-Slavic subfamilies, in which original palatal velar stops became fricatives (as k' > s or š) and labiovelar stops became plain velars (as kw > k).
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. referring to a Proto-Indo-European language group that produced sibilants from a series of palatovelar stops.
Avestan satəm, hundred (a word whose initial sound illustrates the change).(American Heritage® Dictionary of the English Language, Fourth Edition)
Avestan 𐬯𐬀𐬙𐬇𐬩 (satəm, "hundred"), the canonical example of a word that changed Proto-Indo-European palatovelar */ḱ/ into sibilant /s/ by the 3rd millennium BCE, as opposed to Latin centum ("hundred"). (Wiktionary)