Flap is a sound articulated in such a way that one articulator, usually the tongue tip, is drawn back and then allowed to strike against another articulator in returning to its rest position (cf. Ladefoged (1975))
in American English /r/ in mirror or /t/ in dirty are often flaps.
- Ladefoged 1975. A course in phonetics, Harcourt Brace Jovanovich, New York.