The Collaborative International Dictionary of English v.0.48:

Limonin \Li*mo"nin\ (l[-i]*m[=o]"n[i^]n), n. [From NL. Citrus
   Medica, var. Limonum, the scientific name of the lemon.]
   (Chem.)
   A bitter, white, crystalline substance found in orange and
   lemon seeds.
   [1913 Webster]