flatten

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To make flat or flatter.
2. v. To knock down; lay low: The boxer was flattened with one punch.
3. v. To become flat or flatter.

Century Dictionary and Cyclopedia

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

Wiktionary

1. v. transitive To make something flat or flatter.
2. v. 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.
3. v. transitive To knock down or lay low.
4. v. intransitive To become flat or flatter.
5. v. intransitive To be knocked down or laid low.
6. v. music To lower by a semitone.

GNU Webster's 1913

1. v. To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.
2. v. To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit.
3. v. To make vapid or insipid; to render stale.
4. v. (Mus.) To lower the pitch of; to cause to sound less sharp; to let fall from the pitch.
5. v. To become or grow flat, even, depressed, dull, vapid, spiritless, or depressed below pitch.

WordNet 3.0

1. v. lower the pitch of (musical notes)
2. v. make flat or flatter
3. v. become flat or flatter