The act of interspecies sex. Wikipedia has it:
In Larry Niven‘s Ringworld novels, rishathra is “sexual practice outside one’s own species but within the intelligent hominoids.” It is not generally considered a taboo and is often used by the myriad hominids of the Ringworld as a way of sealing agreements, such as trade contracts and peace treaties. Humans, though not native to the Ringworld, share a common descent with the hominids of the Ringworld and may participate freely in rishathra.
Although the term was first introduced in the direct sequel to Ringworld, main character Louis Wu wasn’t exactly a stranger to the dirty deed by the time it was named. Another famous practitioner is Captain Kirk. Getting past the unfamiliar scents and pheromones involved, rishathra may be preferable to prude, human-on-human sex because you’re not likely to catch a sexually transmitted disease, even if your alien lover has one, and the risk of impregnation is practically zero. Hey, who says you can’t have your cake and eat it, too?
Read the entire Wikipedia entry here.
See also: http://en.wikipedia.org/wiki/Sex_in_science_fiction