Also called umlaut, this was a process that historically affected the vowels of some words in Germanic languages. In modern English, the effects of umlaut are most easily seen in certain plural forms (the plurals of "goose, foot, man, mouse, louse" are the i-mutated "geese, feet, men, mice, lice").
It is called i-mutation because originally, this change occurred specifically when the following syllable contained the sound /i/, /iː/ or /j/. This conditioning sound was often lost later on, leaving only the mutated vowel.
Aside from the singular-plural noun pairs listed above, we also can see i-umlaut in some other pairs of related words (e.g. foul and filth, strong and strength, old and elder).
Resources:
- Wikipedia article on Germanic umlaut