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]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "flatten":
   align, assimilate to, balance, beat down, bend, blow down, break,
   break down, bring down, bulldoze, burn down, cast down, chop down,
   conquer, crush, cut down, dab, damp, demolish, disentangle, down,
   drag, dress, dub, equalize, equilibrize, even, extend, fell, floor,
   flush, grade, grease, ground, harmonize, harrow, homogenize,
   humble, knock, knock down, knock out, knock over, lay, lay down,
   lay flat, lay level, lay low, lay out, level, lubricate,
   make uniform, master, mow, mow down, normalize, oil, override,
   plane, planish, plaster, press out, prostrate, pull down, put down,
   put straight, quell, rase, raze, rectify, reduce, regularize,
   regulate, ride down, roll, roll flat, set straight, shave, sit up,
   smash, smooth, smooth down, smooth out, smoothen, stabilize,
   stand up, standardize, steamroll, steamroller, stereotype,
   straighten, straighten out, straighten up, subdue, subjugate,
   suppress, symmetrize, take down, tear down, throw down,
   trample down, trample underfoot, tread underfoot, unbend, uncurl,
   uniformize, unkink, unsnarl, vanquish



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.?


The Free On-line Dictionary of Computing (30 December 2018):

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]