Wiktionary
down tack
n. (context phonetics English) The http://en.wikipedia.org/wiki/IPA diacritic of primary articulation: < ˕ >, used to denote a lowered phone; for vowels, the diacritic denotes a slightly more open utterance; for consonants, it moves the point of articulation one step forward in this chain: nasal → plosive → fricative → approximant → trill → tap, flap → lateral fricative → lateral approximant → lateral flap.