Definition of "lenition"
lenition
noun
countable and uncountable, plural lenitions
(phonetics, phonology) A weakening of articulation causing a consonant to become lenis (soft).
Quotations
One of these processes, the process of T-Lenition, is extremely common, even though it takes place only when the input consonant is adjacent to a small number of affixes. In this change, a stopped consonant, [p t k b d g], becomes a fricative, [s, z, š, ž]. This process is called lenition, or weakening.
2001, Robert Stockwell, Donka Minkova, English Words: History and Structure, page 104
Environments are an essential part of any discussion of lenition. Textbooks often describe lenition as occurring in the weak intervocalic or word-final environments. The canonical examples of lenition given earlier in (1) through (3) all occur either between vowels or between sonorants.
2001, Lisa M. Lavoie, Consonant Strength: Phonological Patterns and Phonetic Manifestations, page 7
As for Goidelic languages, the situation is clearer because Lenition III in this subfamily consisted in losing the same property as the first two lenitions, namely stopness.
2008, Krzysztof Jaskula, Celtic, Joaquim Brandão de Carvalho, Tobias Scheer, Philippe Ségéral (editors), Lenition and Fortition, Studies in Generative Grammar: 99, page 347