What is an organism that lives on or in dead organic matter called?

The term that accurately describes an organism living on or in dead organic matter is "saprotroph." Saprotrophs, which include many fungi and bacteria, play a critical role in ecosystems by breaking down and decomposing organic material, thus recycling nutrients back into the soil. They obtain their nutrients by secreting enzymes that decompose complex organic materials into simpler compounds that can then be absorbed and utilized.

While "decomposer" is a broader term that encompasses saprotrophs, the specific label of saprotroph focuses on their nutrient absorption method and symbiotic relationship with decaying matter. Scavengers, on the other hand, are typically animals that consume already dead organisms, rather than living off of them in the same way saprotrophs do. Herbivores are organisms that primarily feed on plants and do not interact with dead organic matter in the same context. Thus, "saprotroph" is the most precise term for organisms that thrive on decomposing organic material.