Flat·ten v. t. [imp. & p. p. Flattened p. pr. & vb. n. Flattening.]
 1. To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.
 2. To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit.
 3. To make vapid or insipid; to render stale.
 4. Mus. To lower the pitch of; to cause to sound less sharp; to let fall from the pitch.
 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.