compensatory lengthening

noun, Historical Linguistics.

1.

the lengthening of a vowel when a following consonant is weakened or lost, as the change from Old English niht[nikht] /nɪxt/ (Show IPA) to night[nahyt] /naɪt/ (Show IPA) with loss of [kh] /x/ (Show IPA) and lengthening of [i] /ɪ/ (Show IPA) to a vowel that eventually became [ahy] /aɪ/ (Show IPA).