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Lenition is a kind of consonant mutation that appears in many languages. The name lenition appears especially, but not exclusively, in the context of Celtic languages like Welsh, in which it's a pervading feature.
Lenition means 'softening' or 'weakening' (from Latin lenis, as in the root of 'lenient'), and it refers to the change of a consonant considered 'strong' into one considered 'weak' (fortis → lenis). The criteria for deciding whether a consonant is one or the other kind are variable, but in general, the scale goes like this: voiceless stopss (/p t k/) → voiced stops (/b d g/) → voiced fricatives (/v D G/).
Synchronical lenition happens in the Celtic languages, where it's conditioned by grammatical rules (for example, the initial consonant of a noun is lenited, if applicable, when preceded by an article). Diachronical lenition is found, for example, in the change from Latin into Spanish, where word-medial intervocalic voiceless stops (/p t k/) changed into their voiced counterparts (vita → vida, caput → cabo, caecus → ciego).