symbiosis

n. In biology, the living together of two distinct species. It is most commonly used to describe the situation in which both species benefit from living together as humans benefit from having Lactobacillus in their gut which allows them to digest milk, while drinking milk feeds the Lactobacillus. The term may also include situations in which there is benefit to only one of the species involved and it is either harmful or neutral to the other species and some situations in which it is harmful to both species, as in competition.