(phonology) A phonemic merger where the vowels in "pin" and "pen" are pronounced the same before /n/ and /m/, making "pin" and "pen" homonyms.
2011, Scott F. Kiesling, Linguistic Variation and Change, page 141:
Before we get to those, however, let us consider how we might discover a vowel merger taking place (and most work in this area has been on vowel merger or on conditioned vowel mergers, such as the pin–pen merger in the US south or the pool–pull merger in Utah, discussed below).