The Collaborative International Dictionary of English v.0.48:

Betel nut \Be"tel nut`\
   The nutlike seed of the areca palm, chewed in the East with
   betel leaves (whence its name) and shell lime.
   [1913 Webster]

WordNet (r) 3.0 (2006):

betel nut
    n 1: seed of betel palm; chewed with leaves of the betel pepper
         and lime as a digestive stimulant and narcotic in
         southeastern Asia [syn: betel nut, areca nut]