In general a wave is a moving self-sustained disturbance of a medium, and that medium can be either a field (e.g. the gravitational field) or a substance (a solid or fluid). Here, the focus is on waves in a material media, and these are known as mechanical waves .

When a wavetrain of any kind is created in some real, finite medium (whether a string, a drum or the earth itself), it will propagate outward until it encounters a end or boundary. There, some fraction of the wave-energy will usually be reflected backward, and if the original disturbance is sustained, the medium will quickly fill with waves traveling back and forth over one another. These disturbances will combine or interfere to form a steady-state distribution of energy known somewhat paradoxically as