A flap (also: tap or single tap trill) is the term for a speech sound in which one articluator briefly makes contact with another articutlaor without aspiration. This term is most frequently used to refer secifically to an alveolar flap that occurs as an allophone of /t/.
In American English, a flap is produced when the phoneme /t/ occurs between two vowels and the first is stressed (e.g., butter /bʌˈtɚ/). Other examples include: cattle, dirty, Katie, gutted, heated, waiter. The use of a flap does not occur if a consonant occurs next to /t/ (e.g., hatmaker, helter) or when the vowel that followed the /t/ is stressed (e.g., gratuity, maternal, Italian). In England, on the other hand, the flap is not likley to appear in dialects even in the context prescribed above.