The act of having sexual intercourse with one who is strange, foreign, or alien.
In science fiction, and colloquially, xenocopulation usually refers to sexual relations with beings from other worlds, i.e., with aliens/extraterrestrials.
Several science fictions are rife with instances of xenocopulation, and in some cases, particularly Star Trek, have examples of hybrid offspring resulting from xenocopulation.
Researcher: "Great Scott! We've made contact with aliens!"
Scientist: "This is sure to be the beginning of a new era! Imagine the knowledge we'll share!"
Researcher: "Sharing the stories of times long since past!"
Engineer: "Those things are nice and all, but will the xenocopulation be any good?"
Spock: "As captain Kirk is preoccupied with xenocopulating with yet another alien female, I have the bridge in his absence."