Lenition

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.

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary