Meaning of compensatory lengthening

compen'satory length'ening

Pronunciation: [key]
— Historical Ling. Historical Ling.
  1. the lengthening of a vowel when a following consonant is weakened or lost, as the change from Old English niht (nikht) to night (nīt), with loss of (kh) and lengthening of (i) to a vowel that eventually became (ī).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.