The spectral index of an observed electromagnetic-radiation source is a scalar measure describing an aspect of its spectrum, treating the observed spectrum as a power law. It is specifically the exponent of the frequency that produces a term proportional to the observed radiant flux at each frequency, i.e.,
S ∝ να
(Some use the term spectral index to mean the negative this exponent. Also, spectral index has been used to refer to an exponent of the wavelength rather than of the frequency. Some papers deliberately quote the formula they use to avoid confusion. Comments below use the above definition/formula.)
The spectral index must be limited to a finite range/regime: otherwise it would imply infinite energy emission and a frequency range limited only by zero, with no upper limit. In actual practice, a spectral index is used to describe the spectrum viewed over a portion of the spectrum's full range, and describes an approximation of the slope characteristics of radiant-flux-per-unit-frequency over that particular portion. Thermal radiation, which clearly is not a power law over the whole spectrum, given its peak strength in the middle, does approximate a power law of α = 2 over the lower frequencies, as per the Rayleigh-Jeans law. If an observation is made in the regime where this holds true (called spectrum's the Rayleigh-Jeans regime), then thermal radiation is a possibility and parameters of the spectrum indicate the temperature it suggests.
Other spectral indexes suggest other types of EMR production, e.g., synchrotron radiation or bremsstrahlung.
The term emissivity index (β) refers to the spectral index minus two, basically comparing it to the low-frequency end of thermal emission.
Radio astronomy makes much use of the spectral index, and the term radio spectral index is sometimes used to specify the spectral index in the radio regime.