The Collaborative International Dictionary of English v.0.48:

Flatten \Flat"ten\, v. i.
   To become or grow flat, even, depressed, dull, vapid,
   spiritless, or depressed below pitch.
   [1913 Webster]

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):

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, flatten out]
    3: lower the pitch of (musical notes) [syn: flatten, drop]
       [ant: sharpen]

The Free On-line Dictionary of Computing (19 January 2023):

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]


The Jargon File (version 4.4.7, 29 Dec 2003):

flatten
 vt.

    [common] To remove structural information, esp. 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.”