What is the definition of a substance that aims to prevent or control pests?

A substance that aims to prevent or control pests is best defined as a pesticide. Pesticides are a broad category that encompasses various types of chemicals designed specifically to target and manage different types of pests, including insects, weeds, fungi, and diseases.

Pesticides can come in several forms, including insecticides for managing insect pests, herbicides for controlling unwanted plants (weeds), and fungicides for preventing or eliminating fungal infections. The key aspect that distinguishes pesticides is their overarching goal of pest management, which involves reducing the population of harmful organisms to minimize damage to crops, landscapes, and ornamental plants.

The term "pest control" refers more generally to the practices and methods used to manage pests, which might include cultural, biological, and chemical means, but it does not specifically define a substance designed for that purpose. Herbicides and fungicides are specific types of pesticides, hence they only address particular categories of pests rather than the broad spectrum that “pesticide” covers. Therefore, the definition that encompasses all forms of pest prevention and control most accurately aligns with the term pesticide.