A whisker is a thick, bristly hair that grows on an animal's face. Your cat's long whiskers actually help her to navigate in the dark.
You can also call a human's beard whiskers, but the word usually means the stiff hairs on the snout of an animal like a fox, a walrus, or a rat. In fact, most mammals have whiskers — and humans are the only primates who don't have them. This noun can also mean "a tiny amount," like when a disappointed voter says, "The other guy won by a whisker."