The medium of a wave is any substance that carries the wave, or through which the wave travels. Ocean waves are carried by water, sound waves are carried by air, and the seismic waves of an earthquake are carried by rock and soil.
A wave is a disturbance travelling through matter or space, transferring energy from one place to another. Mechanical waves, such a sound waves and water waves, carry energy through a medium, but the molecules of medium itself are generally not permanently displaced. Electromagnetic radiation, which includes light and radio waves, sometimes behaves as waves and sometimes as particles called photons. Electromagnetic waves can travel without a medium.