Wavelength divided by the length of the spatial domain, or equivalently the number of times a wave has the same phase over the spatial domain. The domain might be 2*PI for the nondimensional case, or 2*PI*R*COS(PHI) for an atmospheric wave, where R is Earth's radius and PHI is latitude.
Obviously, wavenumber is the spatial analogue of frequency. Wavenumber and frequency can be obtained from grid point data by application of a Fourier transform in space or time, respectively. In the atmospheric sciences, wavenumber-frequency diagrams are a common way of visualizing atmospheric waves.