1) Unit of frequency 2) Wave mechanic 
The number of wave crests per unit distance. Wavenumber is the reciprocal of wavelength. Wavenumbers are used extensively in infrared spectroscopy, and usually have units of cm-1. 
In the physical sciences, the wavenumber (also wave number) is the spatial frequency of a wave, either in cycles per unit distance or radians per unit distance. It can be envisaged as the number of waves that exist over a specified distance (analogous to frequency being the number of cycles or radians per unit time). Because of the use of this ter...
(σ) Type: Term Pronunciation: wāv′nŭm-bĕr Definitions: 1. The number of waves per centimeter (cm−1), used to simplify the large and unwieldy numbers heretofore used to designate frequency.
Wavenumber describes the spatial variation of waves, phase change per unit distance.