Either of the two partners involved in such a single sexual encounter.
A single sexual encounter between two individuals, where at least one of the partners has no immediate intention or expectation of establishing a longer-term sexual or romantic relationship. As the phrase implies, the relationship lasts for only one night.
An occasion when a performer or team of them (especially in vaudeville) expects to perform at a theater for a single evening.
One's exclusive romantic partner.
A person with whom one has an important bond of some kind.