1. To make something flat or flatter.
    As there was a lot of damage, we chose the heavy roller to flatten the pitch.
    Mary would flatten the dough before rolling it into pretzels.
  2. To press one's body tightly against a surface, such as a wall or floor, especially in order to avoid being seen or harmed.
  3. To knock down or lay low.
    The prize fighter quickly flattened his challenger.
  4. To become flat or flatter.
  5. To be knocked down or laid low.
  6. To lower by a semitone.
  7. To make vapid or insipid; to render stale.
  8. To combine (separate layers) into a single image.

The above text is a snippet from Wiktionary: flatten
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