1. The number of waves per unit distance perpendicular to a wavefront, that is, the reciprocal of the wavelength. It equals , that is, wavenumber k is to as frequency f is to angular frequency . (Some authors define as the wavenumber.)
2. Spatial frequency, the number of wave cycles per unit of distance in a given direction (direction of the spread); apparent wavenumber. Specifically, the reciprocal of the apparent wavelength along the spread direction:
where f=frequency and Va is apparent velocity. If a wavefront makes the angle with the given direction,
where V is the actual velocity of the wavefront. A wavenumber of zero indicates a wavefront striking a line of detectors simultaneously. See f-k plot.
3. See propagation constant.