Xenobiotic: A compound that is foreign to a biological system.

Xenobiotics: Synthetic chemicals believed to be resistant to environmental degradation. A branch of biotechnology called bioremediation is seeking to develop biological methods to degrade such compounds.

Xenotransplantation: Any procedure that involves the transplantation, implantation, or infusion into a human recipient of either (a) live cells, tissues, or organs from a nonhuman animal source, or (b) human body fluids, cells, tissues or organs that have had contact with live nonhuman animal cells, tissues, or organs outside the organism.

Xerophyte: plant adapted to growth in arid conditions.