1. An organism whose body provides nourishment and shelter for a parasite (see parasitism) or a parasitoid. A definitive (or primary) host is one in which an animal parasite becomes sexually mature; an intermediate (or secondary) host is one in which the parasite passes the larval or asexual stages of its life cycle.

2. An organism that lives in close association with an inquiline (see inquilinism).

3. (in genetics) A cell or organism into which foreign DNA is introduced during gene cloning.