1. The surgical removal of an organ or tissue from one species and transplanting it into a member of a different species, for example: the use of a baboon heart in a human being.
2. The surgical transfer of cells, tissues, or especially whole organs from one species to another.

The rationale for xenotransplantation is the short supply of human organs for transplantation.

The first to show that nonhuman organs could be transplanted to humans and function for a significant period of time was Dr. Keith Reemtsma (1925-2000).

At Tulane University in New Orleans, Dr. Reemtsma in 1963 and 1964 gave chimpanzee kidneys to five patients in the first chimpanzee-to-human transplants. The recipients died (of infection) from eight to sixty-three days after receiving a chimpanzee kidney.

Then, in 1964, Reemtsma transplanted a kidney from a chimpanzee to a 23-year-old teacher. She lived with it for nine months until succumbing to overwhelming infection.

Xenotransplantation is synonymous with "cross-species transplantation".