trap-bath split

Short definition from Wiktionary

Noun

1. 1
   A vowel split in some varieties of English (such as British English as spoken in southern England, American English spoken in Boston, and Australian, New Zealand and South African English) in which the phoneme /æ/ (AHD: ă), as pronounced in other varieties of English, is pronounced in some words as /ɑː/ (AHD: ä).