In physics, a medium is a material or substance that transfers energy or light from one substance to another, from one location to another, or from one surface to another. A medium serves as a carrier for any type of energy, including sound waves, light, and heat, which can be transferred through it. The term "medium" implies that the wave is carried by a substance or material.

Examples of mediums in physics include:

• Transmission medium: Any material substance that can propagate waves or energy, such as air, water, or glass.
• Growth medium: An object in which microorganisms or cells experience growth, such as agar in a petri dish.
• Porous medium: A material that allows fluid to pass through it, such as sand.

It is important to note that the term "medium" can also refer to other things outside of physics, such as a way of communicating information and news to people, or a practitioner of mediumship who communicates with spirits of the dead.