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.

Flatten \Flat"ten\, v. i.
   To become or grow flat, even, depressed dull, vapid,
   spiritless, or depressed below pitch.

Source : WordNet®

flatten
     v 1: make flat or flatter; "flatten a road"; "flatten your
          stomach with these exercises"
     2: become flat or flatter; "The landscape flattened" [syn: {flatten
        out}]
     3: lower the pitch of (musical notes) [syn: {drop}] [ant: {sharpen}]

Source : Free On-Line Dictionary of Computing

flatten
     
        To remove structural information, especially to filter
        something with an implicit tree structure into a simple
        sequence of leaves; also tends to imply mapping to
        {flat ASCII}.  "This code flattens an expression with
        parentheses into an equivalent {canonical} form."
     
        [{Jargon File}]