The (natural or surgical) union of parts of two organisms, especially in such a way as to cause them to share their vascular systems. Organisms so joined then are called parabionts.
A form of symbiosis, typically among ants, in which different species share common nest galleries but maintain distinct broods and do not practice mutualism other than incidentally.
An alien form of life within an organism, commonly invasive, such as a growing cancer.