Biological activity refers to the effects that a substance has on living organisms, including humans, animals, plants, and microorganisms. This can include a wide range of effects, such as the ability to stimulate or inhibit biological processes, cause changes in cellular structures, or impact the overall health and function of an organism. Researchers in this field study the biological activity of various substances, such as drugs, toxins, natural products, and chemicals, to understand their mechanisms of action and potential applications in medicine, agriculture, and environmental science. This research area is critical for drug discovery, toxicology, and understanding the interactions between living organisms and their environment.