For a monochromatic wave, "wavelength" is the distance between two successive points in a direction perpendicular to the wavefront where at a given instant the phase differs by $2\pi$. The wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats. The SI unit of wavelength is the meter.
Applicable units are those of quantitykind:Length
http://en.wikipedia.org/wiki/Wavelength
$\lambda = \frac{c}{\nu}$, where $\lambda$ is the wave length, $\nu$ is the frequency, and $c$ is the speed of light in the medium.