lenite

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To undergo an increase in sonority or become lenis. Said of consonant sounds, as when (p) changes to (b), (b) to (v), or (v) to (w).
2. v. To cause (a consonant sound) to lenite.

Wiktionary

1. v. Celtic languages To cause (a consonant) to undergo lenition.
2. v. Celtic languages, of a consonant To undergo lenition.