The wave vector (or k vector) of a plane wave is a vector which points in the direction in which the wave propagates. It is always perpendicular to the wavefronts. Its magnitude is the wavenumber as defined by
where λ is the wavelength in the medium (not the vacuum wavelength).
In non-isotropic media, the direction of energy flow can deviate from that of the wave vector. This phenomenon is called spatial walk-off.