2001, Robert Stockwell, Donka Minkova, English Words: History and Structure, page 104:
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, Lisa M. Lavoie, Consonant Strength: Phonological Patterns and Phonetic Manifestations, page 7:
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.
2008, Krzysztof Jaskula, Celtic, Joaquim Brandão de Carvalho, Tobias Scheer, Philippe Ségéral (editors), Lenition and Fortition, Studies in Generative Grammar: 99, page 347,
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.
2011, Naomi Gurevich, 66: Lenition, Marc van Oostendorp, Colin J. Ewen, Elizabeth V. Hume, Keren Rice (editors), The Blackwell Companion to Phonology, Volume III: Phonological Processes, page 1573,
Five general patterns of lenitions – all based to some extent on empirical data – are identified.