|Definition - A word whose internal vowels change to form its plural.|
Example - English has seven such pairs: man/men, louse/lice, tooth/teeth, mouse/mice, goose/geese, foot/feet, and woman/women.
Oxford English Dictionary - The concept's first citation (there is no entry for the exact phrase) is from 1875–6:
"It is extremely probable that all subjunctives originally had mutated vowels."
( H. Sweet in Trans. Philol. Soc. 549 )