The Collaborative International Dictionary of English v.0.48:

Hackberry \Hack"ber`ry\ (h[a^]k"b[e^]r`r[y^]), n. (Bot.)
   A genus of trees (Celtis) related to the elm, but bearing
   drupes with scanty, but often edible, pulp. Celtis
   occidentalis is common in the Eastern United States. --Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Celtis occidentalis
    n 1: large deciduous shade tree of southern United States with
         small deep purple berries [syn: American hackberry,
         Celtis occidentalis]