A stag is another word for a buck, or a male deer. If you spot a family of deer in the woods, the stag is the one with the largest antlers.
There are different terms for male deer, depending on the species, and stag usually describes the largest types of deer. Stags are also commonly seen in paintings of deer hunts. Another way to use the word stag is as a verb meaning "go without a date," especially to a formal party or dance. In the mid-1800's, a "stag party" was a group made up only of men, a term that came to mean a bachelor party.