mutated plural
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 )

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