verb (v)

• make flat or flatter(verb.change)

  Example:
  Flatten a road.
  Flatten your stomach with these exercises.
  

  source: wordnet30
• become flat or flatter(verb.change)

  Example:
  The landscape flattened.
  

  Synonym:
  flatten out
  

  source: wordnet30
• lower the pitch of (musical notes)(verb.change)

  Synonym:
  drop
  Antonym:
  sharpen
  

  source: wordnet30
• To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.(verb)

  source: webster1913
• To become or grow flat, even, depressed, dull, vapid, spiritless, or depressed below pitch.(verb)

  source: webster1913