flatten

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To make flat or flatter.
• intransitive verb To knock down; lay low.
• intransitive verb To become flat or flatter.

from The Century Dictionary.

• Flat; foolish.
• To make flat; reduce to an equal or even surface; level.
• To lay flat; bring to the ground; prostrate.
• To make vapid or insipid; render stale.
• In music, same as flat, 4.
• To deaden or deprive of luster, as a pigment; bring to a smooth surface or even tint, without relief or gradation.
• In optics, to free from curvature or distortion, as the lines of an image projected by a lens.
• To become flat; grow or become even on the surface.
• To become stale, vapid, or tasteless.
• In music, same as flat, 3.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb To become or grow flat, even, depressed, dull, vapid, spiritless, or depressed below pitch.
• transitive verb To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.
• transitive verb To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit.
• transitive verb To make vapid or insipid; to render stale.
• transitive verb (Mus.) To lower the pitch of; to cause to sound less sharp; to let fall from the pitch.
• transitive verb (Naut.) to set it more nearly fore-and-aft of the vessel.
• transitive verb in glass making, a heated chamber in which split glass cylinders are flattened for window glass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive To make something flat or flatter.
• verb reflexive To press one's body tightly against a surface, such as a wall or floor, especially in order to avoid being seen or harmed.
• verb transitive To knock down or lay low.
• verb intransitive To become flat or flatter.
• verb intransitive To be knocked down or laid low.
• verb music To lower by a semitone.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb lower the pitch of (musical notes)
• verb make flat or flatter
• verb become flat or flatter