What is Wavenumber?

Example 34.2 (From a wavenumber to a wavelength)

A band at $\sigma = 2000\,\mathrm{cm}^{-1}$ corresponds to $\lambda = 1/2000 = 5.0 \times 10^{-4}\,\mathrm{cm} = 5.0\,\text{µ}\mathrm{m}$. Infrared spectra usually run from $4000\,\mathrm{cm}^{-1}$ ($2.5\,\text{µ}\mathrm{m}$) to $500\,\mathrm{cm}^{-1}$ ($20\,\text{µ}\mathrm{m}$).