What is "compensatory lengthening"

n. (context linguistics English) The lengthening of a vowel sound which occurs when a following consonant is lost, an extreme form of fusion exhibited for example by many non-rhotic British dialects of English, which drop the /ɹ/ from /ɑɹt/ but compensatorily lengthen the /ɑ/, resulting in /ɑːt/ ("art").

Compensatory lengthening in phonology and historical linguistics is the lengthening of a vowel sound that happens upon the loss of a following consonant, usually in the syllable coda, or of a vowel in an adjacent syllable. Lengthening triggered by consonant loss may be considered an extreme form of fusion (Crowley 1997:46). Both types may arise from speakers' attempts to preserve a word's moraic count.