Lenition means 'softening' or 'weakening' (from Latin lenis = weak), and it refers to the change of a consonant considered 'stronger' into one considered 'weaker' (or fortis → lenis). Common examples include voicing or sonorization, as in [f] → [v]; affrication or spirantization (turning into an affricate or a fricative), as in [t] → [ts] → [s]; debuccalization (loss of place), as in [s] → [h]; degemination, as in [kː] → [k]; deglottalization, such as [k’] → [k], etc. These may occur one after the other in the history of a language. Eventually, consonants may be lost completely, which is the ultimate lenition. Lenition, then, can be seen as a movement on the sonority scale from less to more sonorous, or on a strength hierarchy from stronger to weaker.
Two common lenition pathways are the "opening" type, where the articulation becomes more open with each step,
and the "sonorization" type, which involves voicing as well,
These pathways may become mixed. For example, [kʰ] may spirantize to [x], then sonorize to [ɣ]. However, whereas sounds change easily in these directions, change in the opposite direction (fortition) generally requires a specific triggering environment.
Diachronic lenition is found, for example, in the change from Latin into Spanish, in which the intervocalic voiceless stops first changed into their voiced counterparts , and later into the approximants : vita → vida, caput → cabo, caecus → ciego. A similar development occurred in the Celtic languages, where non-geminate intervocalic voiced consonants were converted into fricatives through lenition, and voiceless stops became voiced (in Welsh, Cornish and Breton). An example of historical lenition in the Germanic languages is evidenced by English-Latin cognates such as pater, tenuis vs. father, thin. The Latin words preserved the original stops, which became fricatives in old Germanic.
Like several Romance languages, many varieties of Sardinian offer an example of sandhi where the rule of intervocalic lenition extends across word boundaries. Since it is a fully active synchronic rule, lenition is not normally indicated in the normal orthography.
|/b/||→ [β]: baca [baka] 'cow' → sa baca 'the cow'|
|/d/||→ [ð]: domu [dɔmu] 'house' → su domu 'the house'|
|/g/||→ [ɣ]: gupu [gupu] 'ladle' → su gupu 'the ladle'|
Synchronic lenition in Scottish Gaelic affects almost all consonants (except /l̪ˠ/ which has lost its lenited counterpart). Changes such as /n̪ˠ/ to /n/ involve the loss of secondary articulation; in addition, /rˠ/ → /ɾ/ involves the reduction of a trill to a tap. The spirantization of Gaelic nasal /m/ to /v/ is unusual among forms of lenition, but is triggered by the same environment as more prototypical lenition. (It may also leave a residue of nasalization in adjacent vowels. The orthography shows this by inserting an h (except after l n r):
|/b̊/||→ /v/||bog /b̊oɡ̊/ → glé bhog|
|/b̊j/||→ /vj/ (before a back vowel)||beò /b̊jɔː/ 'alive' → glé bheò 'very alive'|
|/k/||→ /x/||cas /kas̪/ 'steep' → glé chas 'very steep'|
|/kʲ/||→ /ç/||ciùin /kʲuːnʲ/ 'quiet' → glé chiùin 'very quiet'|
|/d̪̊/||→ /ɣ/||dubh /d̪̊uh/ 'black' → glé dhubh 'very black'|
|/d̥ʲ/||→ /ʝ/||deiseil /d̥ʲeʃal/ 'ready' → glé dheiseil 'very ready'|
|/ɡ̊/||→ /ɣ/||garbh /ɡ̊aɾav/ 'rough' → glé gharbh 'very rough'|
|/ɡ̊ʲ/||→ /ʝ/||geur /ɡ̊ʲiaɾ/ 'sharp' → glé gheur 'very sharp'|
|/m/||→ /v/||maol /mɯːl̪ˠ/ 'bald' → glé mhaol 'very bald'|
|/mj/||→ /vj/ (before a back vowel)||meallta /mjaul̪ˠd̪̊ə/ 'deceitful' → glé mheallta 'very deceitful'|
|/p/||→ /f/||pongail /pɔŋgal/ 'exact' → glé phongail 'very exact'|
|/pj/||→ /fj/ (before a back vowel)||peallagach /pjal̪ˠaɡ̊əx/ 'shaggy' → glé pheallagach 'very shaggy'|
|Loss of secondary articulation|
|/n̪ˠ/||→ /n/||nàdarra /n̪ˠaːd̪̊ərˠə/ 'natural' → glé nàdarra 'very natural'|
|/rˠ/||→ /ɾ/||rag /rˠaɡ̊/ 'stiff' → glé rag 'very steep'|
|/s̪/||→ /h/||sona /s̪ɔnə/ 'happy' → glé shona 'very happy'|
|/ʃ/||→ /h/||seasmhach /ʃes̪vəx/ 'constant' → glé sheasmhach 'very constant'|
|/ʃ/||→ /hj/ (before a back vowel)||seòlta /ʃɔːl̪ˠd̪̊ə/ 'sly' → glé sheòlta 'very sly'|
|/t̪/||→ /h/||tana /t̪anə/ 'thin' → glé thana 'very thin'|
|/tʲ/||→ /h/||tinn /tʲiːnʲ/ 'ill' → glé thinn 'very ill'|
|/tʲ/||→ /hj/ (before a back vowel)||teann /tʲaun̪ˠ/ 'tight' → glé theann 'very tight'|
|/f/||→ Ø||fann /faun̪ˠ/ 'faint' → glé fhann 'very faint'|
|/fj/||→ /j/ (before a back vowel)||feòrachail /fjɔːɾəxal/ 'inquisitive' → glé fheòrachail 'very inquisitve'|
|Reduction of place markedness|
|In the modern Goidelic languages, grammatical lenition also triggers the reduction of markedness in the place of articulation of coronal sonorants (l, r, and n sounds). In Scottish Gaelic, /n/ and /l/ are the weak counterparts of palatal /ɲ/ and /ʎ/.|
|/ɲ/||→ /n/||neulach /ɲial̪ˠəx/ 'cloudy' → glé neulach 'very cloudy'|
|/ʎ/||→ /l/||leisg /ʎeʃɡ̊ʲ/ 'lazy' → glé leisg 'very lazy'|
Sonorization-type lenition is represented by a simple letter switch in the Brythonic languages, for instance carreg 'stone' → y garreg 'the stone' in Welsh. In Irish orthography, it is shown by writing the 'weak' consonant alongside the (silent) 'strong' one: peann 'pen' → bpeann, ceann 'head' → gceann (sonorization is traditionally called 'eclipsis' in Irish grammar).
An example with geminate consonants comes from Finnish, where geminates become simple consonants while retaining voicing or voicelessness (e.g. katto → katon, dubbaan → dubata). It is also possible for entire consonant clusters to undergo lenition, as in Votic, where voiceless clusters become voiced, e.g. itke- → idgön.
If a language has nothing but voiceless stops, other sounds are encountered, as in Finnish, where fricatives are represented by chronemes, approximants, taps or even trills. For example, Finnish used to have a complete set of spirantization reflexes for , though these have been lost in favour of similar-sounding phonemes. In Pohjanmaa Finnish, /ð/ was changed into /r/, thus the dialect has a synchronic lenition of an alveolar stop into an alveolar trill . Furthermore, the same phoneme /t/ undergoes assibilation te → si, e.g. root vete- → vesi and vere-. Here, vete- is the stem, vesi is its nominative, and vere- is the same stem under consonant gradation.