Source : Webster's Revised Unabridged Dictionary (1913)

Flatten \Flat"ten\, v. t. [imp. & p. p. {Flattened}; p. pr. &
   vb. n. {Flattening}.] [From {Flat}, a.]
   1. To reduce to an even surface or one approaching evenness;
      to make flat; to level; to make plane.

   2. To throw down; to bring to the ground; to prostrate;
      hence, to depress; to deject; to dispirit.

   3. To make vapid or insipid; to render stale.

   4. (Mus.) To lower the pitch of; to cause to sound less
      sharp; to let fall from the pitch.

   {To flatten a sail} (Naut.), to set it more nearly
      fore-and-aft of the vessel.

   {Flattening oven}, in glass making, a heated chamber in which
      split glass cylinders are flattened for window glass.

Source : WordNet®

flattened
     adj : having been flattened [syn: {planate}]