What term describes any substance that repels or mitigates pests?

The term that describes any substance that repels or mitigates pests is pesticide. This is a broad category that encompasses various types of products designed to control or eliminate undesirable organisms, including insects, weeds, fungi, and other pests. Pesticides can function in different ways, such as repelling, killing, or disrupting the life cycles of these pests. While the term can be specific to certain types (like insecticides for insects, herbicides for plants, and fungicides for fungi), in this context, pesticide is the appropriate term that encompasses all these functions at a general level.

Herbicides are specifically for controlling weeds, insecticides target insects, and fungicides aim to manage fungal pests. All of these fall under the larger category of pesticides, which is why pesticide is the most accurate answer in defining any substance with the ability to repel or mitigate pests.