A smile in which the lips are parted to reveal the teeth.
"The old fisherman cast his net, hoping to catch more than just a few small fish in the shimmering grin of the morning sun."
A snare; a gin.
to draw back the lips and reveal the teeth, in a smile, grimace, or snarl
"The dog gave an angry grin as it bared its sharp teeth at the intruder."
To smile, parting the lips so as to show the teeth.
"He broke into a wide grin that revealed his entire set of white teeth when he heard the good news."