The Collaborative International Dictionary of English v.0.48:

Cola nut \Cola nut\, Cola seed \Cola seed\ . (Bot.)
   The bitter fruit of Cola acuminata, which is nearly as
   large as a chestnut, and furnishes a stimulant, which is used
   in medicine.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cola nut
    n 1: bitter brown seed containing caffein; source of cola
         extract [syn: kola nut, cola nut]