Compression is a process where the particles of a substance are forced closer together. This can occur due to an increase in pressure, a decrease in temperature, or a combination of both. In the case of a medium such as a gas or liquid, compression involves the particles being forced closer together, resulting in an increase in density.

In a solid medium, compression can cause the particles to move closer together, resulting in an increase in the material's stiffness and hardness. This is because the particles are more closely packed and therefore have less space to move.

Overall, compression refers to the process of reducing the volume of a medium by bringing its particles closer together.