In physics, this refers to a substance through which a wave propagates. This substance can be solid, liquid, gas, or plasma. It is the material that supports the transmission of energy in the form of waves. Examples include air for sound waves, water for water waves, and various materials for electromagnetic radiation.
The presence of this substance is often essential for wave propagation. It determines the wave’s speed and characteristics. Understanding the properties of this substance is crucial in fields such as optics, acoustics, and seismology. Historically, its existence was a key concept in understanding light, though modern physics acknowledges electromagnetic waves can also propagate through a vacuum.