1. Spatial frequency (k); the number of complete wave cycles per unit distance, and the inverse of wavelength (λ), such that k = 1/λ.
2. Propagation constant; in electromagnetic theory, wavenumber is defined as 2π/λ, and k2 = μω(∈ω + iσ) for the time factor eiωt, where μ is the magnetic permeability, ω the angular frequency in radians per second, ∈ the dielectric permittivity, and σ the electrical conductivity.