Rishathra is a noun
used to describe sexual intercourse
of different species
The word was first introduced in Larry Niven's Ringworld and its sequels The Ringworld Engineers, The Ringworld Throne, and Ringworld's Children. It has since spread in the science fiction community much in the same way as grok and fnord.
In Niven's novels, rishathra is practiced as a way of sealing agreements and preventing pregnancy and STDs.
The verb form of the word is rish.