What term describes a naturally occurring poison produced by plants, animals, or microorganisms?

The term that describes a naturally occurring poison produced by plants, animals, or microorganisms is "toxin." Toxins are specific types of toxic substances that are biologically produced and can cause harm to organisms. They play various roles in the natural world, such as defense mechanisms for plants and animals against predators or competition.

When considering the other options, venom refers specifically to toxins that are injected into a victim, often through bites or stings, by animals like snakes and spiders. A toxoid is a modified toxin that has lost its toxicity but can still provoke an immune response, commonly used in vaccines. A pathogen is a microorganism that causes disease, which might produce toxins but is not itself classified as a toxin. Therefore, "toxin" is the most accurate term for the definition provided.