The Collaborative International Dictionary of English v.0.48:

Undecylic \Un`de*cyl"ic\, a. (Chem.)
   Related to, derived from, or containing, the undecyl radical;
   specifically, designating that member of the fatty acids
   which corresponds to undecane, and is obtained as a white
   crystalline substance, C10H21.CO2H.
   [1913 Webster]