First off a wave is the disturbance that transfers energy from place to place. The crest is the highest point on a transverse wave. The trough is the lowest point of the wave. The Compressions are the parts of the longitudinal wave that are closest together. Rarefactions are the parts of the wave that are closest together. Amplitude is the maximum distance from the rest position that the medium moves in a wave. The wavelength is the distance between two parts of a wave. The interference is when one wave hits another wave their amplitudes combine and make a new wave. A medium is the material a mechanical waves moves through. A vacuum is totally empty space.