The Collaborative International Dictionary of English v.0.48:

Thinner \Thin"ner\, n.
   One who thins, or makes thinner.
   [1913 Webster]
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinner
    n 1: a diluting agent [syn: dilutant, diluent, thinner]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "thinner":
   anticoagulant, chromogen, coat, coat of paint, coating, color,
   color filter, color gelatin, colorant, coloring, dead-color,
   diluent, dilutant, dissolvent, dissolving agent, distemper, drier,
   dye, dyestuff, exterior paint, flat coat, flat wash, floor enamel,
   flux, ground, interior paint, liquefacient, liquefier, medium,
   opaque color, paint, pigment, prime coat, primer, priming,
   resolutive, resolvent, solvent, stain, tempera, tinction, tincture,
   transparent color, turpentine, turps, undercoat, undercoating,
   vehicle, wash, wash coat