(phonology) Aphonemic merger in some varieties of English (especially American, Canadian and Scottish English) in which thevowels in words such ashot anddoll (/ɒ/) and in words such aslaw andtalk (/ɔː/) are pronounced identically, making the wordscot andcaughthomophones.
2011, Scott F Kiesling,Linguistic Variation and Change, page81:
One was /au/-monophthongisation, described earlier in Chapter 2, while the second was the so-called low-back merger (LBM), also referred to as thecot–caught merger because words in these two classes are pronounced the same and speakers cannot hear any small differences in their pronunciation, even when they are present.