broken plural

Short definition from Wiktionary

Noun

1. 1
   An irregular plural formed by changing the pattern of consonants and vowels inside the singular form, found in the Semitic languages and other Afroasiatic languages such as Berber. An example in Arabic is مَكَاتِب (makātib, “desks”), the plural form of مَكْتَب (maktab, “desk”).