A wave is an oscillation that transfers energy through a medium or space. There are two types of waves - mechanical waves, which require a medium, and electromagnetic waves, which do not. For any wave, there is a propagation direction where the wave travels and an oscillation direction in which particles of the medium move. If these directions are perpendicular, it is a transverse wave, and if they are parallel, it is a longitudinal wave. The wavelength is the distance between two peaks of a wave and the frequency is the number of waves that pass a fixed point per second.