The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illuminant
    n 1: something that can serve as a source of light

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "illuminant":
   aglow, beaming, beamy, benzine, blushing, bright and sunny,
   burning, candescent, candle, coal oil, electric light bulb,
   electricity, fire, flame, flushing, gas, gasoline, gleaming,
   gleamy, glim, glinting, glowing, illuminating, illuminating gas,
   illuminative, illuminator, illumining, incandescent,
   incandescent body, irradiative, kerosene, lamp, lamping, lantern,
   light, light as day, light bulb, light source, luciferous, lucific,
   luciform, luminant, luminary, luminative, luminiferous,
   luminificent, luminous, lustrous, match, moon, oil, orient,
   paraffin, petrol, petroleum, radiant, rutilant, rutilous, shining,
   shiny, source of light, starbright, starlike, starry, stars,
   streaming, suffused, sun, sunny, sunshiny, taper, torch