In three dimensions, the corresponding plane wave term becomes cos ⁡ ⁡ ( k ⋅ ⋅ r − − ω ω t + φ φ 0 ) , which simplifies to cos ⁡ ⁡ ( k ⋅ ⋅ r + φ φ ) at a fixed time t , where r is the position vector of a point in real space and now k = 2 π π e / λ λ is the wavevector in the three dimensional reciprocal space. (The magnitude of a wavevector is called wavenumber.) The constant φ φ is the phase of the wavefront (a plane of a constant phase) through the origin r = 0 at time t , and e is a unit vector perpendicular to this wavefront. The wavefronts with phases φ φ + ( 2 π π ) n , where n represents any integer, comprise a set of parallel planes, equally spaced by the wavelength λ λ .