[ flat-n ]
See synonyms for flatten on Thesaurus.com
verb (used with object)
  1. to make flat.

  2. to knock down: The boxer flattened his opponent in the second round.

verb (used without object)
  1. to become flat.

Verb Phrases
  1. flatten in, Nautical. flat1 (def. 61).

  2. flatten out, Aeronautics. to fly into a horizontal position, as after a dive.

Origin of flatten

First recorded in 1620–30; flat1 + -en1

Other words for flatten

Other words from flatten

  • flat·ten·er, noun
  • o·ver·flat·ten, verb (used with object)
  • un·flat·tened, adjective

Words Nearby flatten

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use flatten in a sentence

British Dictionary definitions for flatten


/ (ˈflætən) /

  1. (sometimes foll by out) to make or become flat or flatter

  2. (tr) informal

    • to knock down or injure; prostrate

    • to crush or subdue: failure will flatten his self-esteem

  1. (tr) music to lower the pitch of (a note) by one chromatic semitone: Usual US word: flat

  2. (intr foll by out) to manoeuvre an aircraft into horizontal flight, esp after a dive

Derived forms of flatten

  • flattener, noun

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012