Exhibitionism is behaviour by a person that involves the exposure of private parts of their body to another person in a situation when they would not normally be exposed, with the implication that the person exposing him or herself is being aroused by their actions.
As a tag on e621, it refers to images or animations of characters deliberately engaging in sexual activities in view of at least one other character. This can range from a man masturbating in front of a webcam for his girlfriend, to a man who likes to have sex with other men in front of his willing husband, or to a group of people having an impromptu orgy in a public park, in full view of all the strangers wandering through.
Note that if the character(s) watching the sexual act are doing so in order to become aroused themselves, the voyeur tag also applies. If they're watching without being aroused, being watched applies.
The following tags are aliased to this tag: exhibitionist