verb (used with object)
- to make flat.
- to knock down: The boxer flattened his opponent in the second round.
verb (used without object)
- to become flat.
- flatten in, Nautical. flat1(def 61).
- flatten out, Aeronautics. to fly into a horizontal position, as after a dive.
- (sometimes foll by out) to make or become flat or flatter
- (tr) informal
- to knock down or injure; prostrate
- to crush or subduefailure will flatten his self-esteem
- (tr) music to lower the pitch of (a note) by one chromatic semitoneUsual US word: flat
- (intr foll by out) to manoeuvre an aircraft into horizontal flight, esp after a dive