grin

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To smile broadly, often baring the teeth, as in amusement, glee, embarrassment, or other strong emotion.
2. v. To express with a grin: I grinned my approval.
3. n. The act of grinning.
4. n. The facial expression produced by grinning.

Century Dictionary and Cyclopedia

1. 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. The muscles specially concerned in the act are the levator labii superioris and levator anguli oris.
2. 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.
3. To snarl with, as the teeth in grinning.
4. To effect by grinning.
5. n. The act of withdrawing the lips and showing the teeth; hence, a broad smile; especially, a forced, derisive, sardonic, or vacant smile.
6. n. A snare or trap which snaps and closes when a certain part is touched.

Wiktionary

1. n. A smile in which the lips are parted to reveal the teeth.
2. v. To smile parting the lips so as to show the teeth.

GNU Webster's 1913

1. n. A snare; a gin.
2. v. To show the teeth, as a dog; to snarl.
3. v. 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.
4. v. To express by grinning.
5. n. The act of closing the teeth and showing them, or of withdrawing the lips and showing the teeth; a hard, forced, or sneering smile.

WordNet 3.0

1. v. to draw back the lips and reveal the teeth, in a smile, grimace, or snarl
2. n. a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement