laughter

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

• noun The act of laughing.
• noun The sound produced by laughing.
• noun Archaic A cause or subject for laughter.

from The Century Dictionary.

• noun A mode of expressing mirth, consisting chiefly in certain convulsive and partly involuntary actions of the muscles of respiration, by means of which, after an inspiration, the expulsion of the air from the chest in a series of jerks produces a succession of short abrupt sounds, accompanied by certain movements of the muscles of the face, and often of other parts of the body, and, when excessive, by tears: also sometimes applied to any expression of merriment perceivable in the countenance.
• noun A laugh.

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

• noun A movement (usually involuntary) of the muscles of the face, particularly of the lips, with a peculiar expression of the eyes, indicating merriment, satisfaction, or derision, and usually attended by a sonorous and interrupted expulsion of air from the lungs. See laugh, v. i.

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

• noun The sound of laughing, produced by air so expelled; any similar sound.
• noun A movement (usually involuntary) of the muscles of the laughing face, particularly of the lips, and of the whole body, with a peculiar expression of the eyes, indicating merriment, satisfaction or derision, and usually attended by a sonorous and interrupted expulsion of air from the lungs.
• noun archaic A reason for merriment

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

• noun the sound of laughing
• noun the activity of laughing; the manifestation of joy or mirth or scorn