The Collaborative International Dictionary of English v.0.48:

flattened \flattened\ adj.
   1. shaped like a thin sheet.

   Syn: planate.
        [WordNet 1.5 +PJC]

   2. (Biol) flattened laterally along the whole length e.g.
      certain leafstalks or flatfishes.

   Syn: compressed, flat.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Flatten \Flat"ten\ (fl[a^]t"t'n), 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.
      [1913 Webster]

   2. To throw down; to bring to the ground; to prostrate;
      hence, to depress; to deject; to dispirit.
      [1913 Webster]

   3. To make vapid or insipid; to render stale.
      [1913 Webster]

   4. (Mus.) To lower the pitch of; to cause to sound less
      sharp; to let fall from the pitch.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flattened
    adj 1: having been flattened [syn: planate, flattened]