In biology, the primary host (aka definitive host; final host) is the host at which the parasite spends its final developmental stage, and reaches its mature form.
The definitive host refers to the organism wherein the parasite reaches its mature form. Thus, the parasite at this stage is typically capable of reproduction. For example, human tapeworm makes use of human as its definitive host.
Secondary host (aka intermediate host) is the host harbouring a parasite that primarily grows but not to the point of reaching (sexual) maturity.
An intermediate host often acts as vector of parasite to reach its definitive host (where it will become mature). Thus, the intermediate host serves only as a site wherein the parasite spends a particular developmental stage of its life cycle (i.e. larval stage).
For example, some tapeworms make use of cows, pigs, and fish as intermediate hosts. When any of these animals ingests a tapeworm egg, the egg hatches and the larva moves from the intestine to the muscle of the animal where it forms a cyst. Human ingesting a partially cooked or raw meat containing the cyst may eventually harbor the parasite when the larva moves out of the cyst and grows into its mature or reproductive form and begin to reproduce inside the definitive human host.
A host is an organism that is infected with or is fed upon by a parasitic or pathogenic organism (for example, a virus, nematode, fungus).
The term can also be applied, loosely, to a plant supporting an epiphyte. An animal or plant that nourishes and supports a parasite; the host does not benefit and is often harmed by the association. (Medicine) recipient of transplanted tissue or organ from a donor. An organism that a parasite is situated within.