flatten

• POS: Verb (usu participle), Verb (intransitive), Verb (transitive)
• HYPHEN: flat=ten

CIDE DICTIONARY

flatten, v. t. [From Flat, a.].

•  To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.  [1913 Webster]
•  To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit.  [1913 Webster]
•  To make vapid or insipid; to render stale.  [1913 Webster]
•  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.

flatten, v. i. 

 To become or grow flat, even, depressed, dull, vapid, spiritless, or depressed below pitch.  [1913 Webster]

OXFORD DICTIONARY

flatten, v.
1 tr. & intr. make or become flat.
2 tr. colloq. a humiliate. b knock down.

Idiom

flatten out bring an aircraft parallel to the ground.

Derivative

flattener n.

THESAURUS

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

ROGET THESAURUS

flatten

Horizontality

VB be horizontal, lie, recline, couch, lie down, lie flat, lie prostrate, sprawl, loll, sit down, render horizontal, lay down, lay out, level, flatten, prostrate, knock down, floor, fell.

Flatness

VB render flat, flatten, level.