As wavelength increases, R.I. decreases (generally).
It also varies with temperature and pressure.
Therefore refractive index is usually tabulated for a given wavelength, temperature an pressure.
I have never heard that it is called 'optical constant'.
The term optical constant is a historical description of optical parameters without dimension e.g. refractive index is a pure numerical entity (ratio of phase velocity in vacuum and material). The same holds for the transmission, absorption coefficient, dispersion a.s.o. Constant has here only the meaning of a pure number.
For electromagnetic waves at lower than optical frequencies (such as micro and millimeter waves in the frequency range 1 GHz to 100 GHz) propagating through the atmosphere, the refractive index also depends strongly on the water vapor content of the atmosphere.
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