Quantitative metathesis

Quantitative metathesis (also "metathesis of quantity") is a technical phonological term which refers to a situation in which two vowel sounds follow directly one after the other — one being long and the other short — and a transposition of vowel length takes place, such that the formerly-long vowel becomes short, and the formerly-short vowel becomes long.

So the ancient Attic Greek form νεώς [neōs] "of a ship" was derived from earlier Attic-Ionic νηός [nēos] by quantitative metathesis.