Biological activity is the capacity of a substance or physical agent to elicit a measurable effect on living organisms, cells, tissues, or biological systems. As a fundamental concept and area of research, it investigates the interactions between chemical or physical entities and biological targets, quantifying the nature, magnitude, and specificity of the resulting physiological, biochemical, or cellular responses. Its significance lies in understanding mechanisms of action, assessing potential therapeutic or toxic effects, and informing the development of pharmaceuticals, agrochemicals, and environmental safety standards.