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):

hackberry
    n 1: any of various trees of the genus Celtis having
         inconspicuous flowers and small berrylike fruits [syn:
         hackberry, nettle tree]
    2: small edible dark purple to black berry with large pits;
       southern United States [syn: sugarberry, hackberry]