grin

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

• intransitive verb To smile broadly, often baring the teeth, as in amusement, glee, embarrassment, or other strong emotion.
• intransitive verb To express with a grin.
• noun The act of grinning.
• noun The facial expression produced by grinning.

from The Century Dictionary.

• To draw back the lips so as to show the teeth set nearly or quite together, as a snarling dog, or a person in pain or anger.
• Hence To smile with a similar distortion of the features; exhibit derision, stupid admiration, embarrassment, or the like, by drawing back the lips from the teeth with a smiling expression.
• To snarl with, as the teeth in grinning.
• To effect by grinning.
• noun The act of withdrawing the lips and showing the teeth; hence, a broad smile; especially, a forced, derisive, sardonic, or vacant smile.
• noun A snare or trap which snaps and closes when a certain part is touched.

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

• noun obsolete A snare; a gin.
• transitive verb To express by grinning.
• noun The act of closing the teeth and showing them, or of withdrawing the lips and showing the teeth; a hard, forced, or sneering smile.
• intransitive verb To show the teeth, as a dog; to snarl.
• intransitive verb To set the teeth together and open the lips, or to open the mouth and withdraw the lips from the teeth, so as to show them, as in laughter, scorn, or pain.

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

• noun A smile in which the lips are parted to reveal the teeth.
• verb To smile parting the lips so as to show the teeth.

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

• verb to draw back the lips and reveal the teeth, in a smile, grimace, or snarl
• noun a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement