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/ to night [nahyt] /naɪt/, with loss of [kh] /x/ and lengthening of [i] /ɪ/ to a vowel that eventually became [ahy] /aɪ/.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018