The Collaborative International Dictionary of English v.0.48:

Trimesitic \Tri`me*sit"ic\, a. [Pref. tri- + mesitylene + -ic.]
   (Chem.)
   Of, pertaining to, or designating, a tribasic acid,
   C6H3.(CO2)3, of the aromatic series, obtained, by the
   oxidation of mesitylene, as a white crystalline substance.
   [Written also trimesic.]
   [1913 Webster]