Wave delay, also known as wave propagation delay or time delay, refers to the time it takes for a wave to travel from one point to another. This delay is caused by the finite speed of the wave through a particular medium. The speed of a wave depends on the properties of the medium, such as its density, elasticity, and temperature.

Waves are disturbances that travel through a medium, transferring energy from one point to another. They can be classified into different types, such as water waves, sound waves, light waves, and seismic waves.

In the context of waves, a crack can refer to a sudden release of energy that generates a wave. For example, a seismic crack can produce seismic waves that travel through the Earth's interior. Similarly, a crack in a material can produce acoustic waves that can be detected using non-destructive testing techniques.