- The fusion of two embryos to form conjoined twins
- (medicine) An alien form of life within an organism, commonly invasive, such as a growing cancer.
- (physiology) 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)
- (ecology) (ethology) 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.
- A transient physiological state of suspension of obvious vital activities such as to enable an organism to escape the notice of its enemies or to conserve its energy.