father-bother merger

noun1

1. •
   A phonemic merger in English of the vowels /ɑː/ (as in father) and /ɒ/ (as in bother).

   • The first is the father-bother merger, a loss of rounding contrast in the low back vowels by which the originally contrastive /ɒ/ has been subsumed by /ɑː/ into the fused phoneme /ɑ/. — 2018