In linguistics, lenition is a sound change that alters consonants, making them "weaker" in some way. The word lenition itself means "softening" or "weakening" (from Latin lēnis 'weak'). Lenition can happen both synchronically (within a language at a particular point in time) and diachronically (as a language changes over time). Lenition can involve such changes as voicing a voiceless consonant, causing a consonant to relax occlusion, to lose its place of articulation (a phenomenon called debuccalization, which turns a consonant into a glottal consonant like [h] or [ʔ]), or even causing a consonant to disappear entirely.

An example of synchronic lenition is found in most varieties of American English, in the form of tapping: the /t/ of a word like wait [weɪt] is pronounced as the more sonorous [ɾ] in the related form waiting [ˈweɪɾɪŋ]. Some varieties of Spanish show debuccalization of /s/ to [h] at the end of a syllable, so that a word like estamos "we are" is pronounced [ehˈtamoh]. An example of diachronic lenition can be found in the Romance languages, where the /t/ of Latin patrem ("father", accusative) has become /d/ in Italian (an irregular change; compare saeta "silk" > seta) and Spanish padre (the latter weakened synchronically /d/ → [ð̞]), while in Catalan pare, French père and Portuguese pai historical /t/ has disappeared completely.

In some languages, lenition has been grammaticalized into a consonant mutation, which means it is no longer triggered by its phonological environment but is now governed by its syntactic or morphological environment. For example, in Welsh, the word cath "cat" begins with the sound /k/, but after the definite article y, the /k/ changes to [ɡ]: "the cat" in Welsh is y gath. This was historically due to intervocalic lenition, but in the plural, lenition does not happen, so "the cats" is y cathod, not *y gathod. The change of /k/ to [ɡ] in y gath is thus caused by the syntax of the phrase, not by the modern phonological position of the consonant /k/.

The opposite of lenition, fortition, a sound change that makes a consonant "stronger", is less common, but Breton and Cornish have "hard mutation" forms which represent fortition.

Lenition involves changes in manner of articulation, sometimes accompanied by small changes in place of articulation. There are two main lenition pathways: opening and sonorization. In both cases, a stronger sound becomes a weaker one. Lenition can be seen as a movement on the sonority hierarchy from less sonorous to more sonorous, or on a strength hierarchy from stronger to weaker.

In examples below, a greater-than sign indicates that one sound changes to another. The notation [t] > [ts] means that [t] changes to [ts].

The sound change of palatalization sometimes involves lenition.

Lenition includes the loss of a feature, such as deglottalization, in which glottalization or ejective articulation is lost: [kʼ] or [kˀ] > [k].