A gain medium is a material or substance that exhibits optical gain, meaning it can amplify light signals passing through it. Gain mediums are commonly used in lasers to enhance the intensity of the emitted light. They are typically made of solid-state materials, gases, or liquids that can be excited by an external energy source to create a population inversion and stimulate the emission of photons. Gain mediums play a crucial role in various applications such as telecommunications, medical devices, manufacturing, and research.