per meter

Wavenumber is the answer to how many waves fit in a metre, and it is the natural currency wherever wavelength gets awkwardly small. Green light near 550 nm packs about 1.8 million cycles into a metre; working spectroscopists shift to the reciprocal centimetre, a hundred times smaller, to keep the numbers readable, and that unit has its own page here. The same reciprocal metre also serves as an attenuation or absorption coefficient, stating what fraction of a beam is lost per metre of path.

The trap in this unit is a silent factor of $2\pi$. Spectroscopy defines wavenumber as $\tilde{\nu} = 1/\lambda$, while physics texts work with the angular wavenumber $k = 2\pi/\lambda$, and both are quoted in reciprocal metres with nothing in the unit to tell them apart. A formula expecting one and fed the other is wrong by 6.28, large enough to matter and small enough to survive a casual sanity check. To move a wavenumber into the site's solvers, invert it to a wavelength for the wave speed relation, or multiply by the speed of light to get the frequency the photon energy solver wants; both steps assume the spectroscopic convention without the $2\pi$.

The most precisely known number ever expressed in this unit is the Rydberg constant, 10,973,731.568 reciprocal metres and change. It sets the wavenumber of every line in the hydrogen spectrum, and decades of laser spectroscopy have pinned it to about twelve significant figures, among the most exactly measured quantities in all of physics.