What do you call an organism that lives on or within another organism at its expense?

The term used to describe an organism that lives on or within another organism at its expense is "parasite." A parasite benefits by deriving nutrients and other advantages from the host organism, often to the detriment of the host's health. This relationship can lead to diseases or other negative effects on the host, as the parasite may consume resources or evade the host's immune system.

In contrast, pathogens, which are often synonymous with the term parasite in common usage, specifically refer to microorganisms that cause disease. However, not all parasites are pathogens if they do not necessarily cause disease but still exploit the host.

Symbionts indicate organisms that have a more mutually beneficial relationship with their hosts. Commensals refer to organisms that benefit from their host without causing significant harm or benefit to the host. In summary, the definition and nature of the relationship established as a parasite highlight the importance of understanding how these interactions impact both the host and the parasite itself.