Wiktionary
n. Sex between any two (or more) humanoids of different species in science fiction.
Wikipedia
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.
Rishathra is also used as a form of birth control. Because it is impossible to impregnate a member of another species, sexual intercourse with such an individual provides sexual release without the risk of pregnancy. The issue of sexually transmitted diseases is not explored in the Ringworld novels until the last book, Ringworld's Children in which it is briefly mentioned that Ringworld STDs cannot jump the species barrier.
The (non-sentient) "Vampire" species of hominid on Ringworld uses rishathra to obtain prey. A powerful pheromone is released that brings any compatible hominid into an extreme state of arousal. The victim immediately attempts to mate with the nearest compatible hominid, usually the vampire. During this mating, the vampire gains sustenance by draining the victim's blood.
The "Ghoul" species of hominid on Ringworld are generally not permitted to "rish" with members of another species. This is due to their place in the Ringworld ecosystem as carrion eaters, specifically the dead bodies of other hominid races, a practice that while accepted by other hominids prohibits such close relations.
The "Water People" species of hominid on Ringworld are not able to rish with most other hominids due to the requirement that such activities take place underwater. Air breathers are typically not able to hold their breath for sufficient time to allow the act of sexual intercourse to take place. (This does not seem to disappoint them, however.)
Usage examples of "rishathra".
And several hundred Grass Giant males were expecting—“I couldn’t handle rishathra now,” Vala said.
They shared rishathra with the species they met, but mating was something else.
What point in enlarging upon a guest’s skill at rishathra, for a species that couldn’t do that at all?
He clearly wasn’t interested in rishathra, or Ghouls, either, Vala thought.
And the great thing about variety was that rishathra wouldn’t work at all if it required an elaborate dance.
Discussions involving physiognomy or rishathra could be answered by pointing.
Morning was Sawur’s time for rishathra, and Louis needed this in the worst way.
Vala hardly needed to hear the shouts of her companions: it was celebration time, rishathra time.
The Grass Giants had begun exploring each other, outside the rishathra pattern—as she and Kay were—and Tegger and, Warvia were talking, just talking.
The only Red Herder woman who ever engaged in rishathra, and all his for the night.
When visitors tell of the vampire slayers who learned rishathra, that will be us.
The tale tells of heroes of their own species who invented rishathra and ended war, and it tells that their own species has practiced rishathra ever since.
No, neither he nor Chmeee had ever practiced rishathra, though there was great variety among his species.
I had thought to lend him my wife Reeth, who has practiced Rishathra before.
Like rishathra, or real mating, without needing to love anyone but yourself and without needing to stop.