1.Definition: a blemish resulting from a break without complete separation of the parts
"There was a crack in the mirror"
2.Definition: a chance to do something
5.Definition: a narrow opening
"He opened the window a crack"
6.Definition: a sudden sharp noise
"The crack of a whip", "He heard the cracking of the ice", "He can hear the snap of a twig"
8.Definition: the act of cracking something