Source : Webster's Revised Unabridged Dictionary (1913)

Illuminant \Il*lu"mi*nant\, n. [L. illuminans, -antis, p. pr. of
   illuminare.]
   That which illuminates or affords light; as, gas and
   petroleum are illuminants. --Boyle.

Source : WordNet®

illuminant
     n : something that can serve as a source of light