Definition of flatten

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