What is the term for an organism that lives on or in another organism (its host) for most of its life cycle?

The term that describes an organism which lives on or in another organism (its host) for most of its life cycle is a parasite. Parasites derive benefits at the expense of their hosts, often harming them in the process. This relationship is characterized by the parasite's dependence on the host for survival, nourishment, and often the completion of its life cycle.

In contrast, commensals are organisms that may live on or in a host but do not harm it, often benefiting from the relationship while the host remains unaffected. Symbionts can refer to both parasitic and mutualistic relationships, capturing a broader range of interactions between different species, but they do not specifically denote the harmful dependency typical of parasitism. Pathogens specifically refer to organisms, usually microbes, that cause disease in their hosts, which is a more narrow definition that focuses solely on disease causation rather than the broader parasitic lifestyle.