A receptor is a specialized structure or protein that can detect and respond to a specific stimulus, such as a chemical substance, light, or pressure, in the environment or in the body. Receptors are often located on the surface of cells or embedded within cell membranes and are part of the sensory system that allows organisms to interact with their surroundings and respond to changes in the environment. When a receptor is activated by its specific stimulus, it triggers a series of physiological responses or signals within the organism that help it to adapt and survive.