The Collaborative International Dictionary of English v.0.48:

Locust tree \Lo"cust tree`\n. [Etymol. uncertain.] (Bot.)
   A large North American tree of the genus Robinia (Robinia
   Pseudacacia), producing large slender racemes of white,
   fragrant, papilionaceous flowers, and often cultivated as an
   ornamental tree. In England it is called acacia.
   [1913 Webster]

   Note: The name is also applied to other trees of different
         genera, especially to those of the genus Hymen[ae]a,
         of which Hymen[ae]a Courbaril is a lofty, spreading
         tree of South America; also to the carob tree
         (Ceratonia siliqua), a tree growing in the
         Mediterranean region.
         [1913 Webster]

   Honey locust tree (Bot.), a tree of the genus Gleditschia
      ) Gleditschia triacanthus), having pinnate leaves and
      strong branching thorns; -- so called from a sweet pulp
      found between the seeds in the pods. Called also simply
      honey locust.

   Water locust tree (Bot.), a small swamp tree (Gleditschia
      monosperma), of the Southern United States.
      [1913 Webster]

WordNet (r) 3.0 (2006):

locust tree
    n 1: any of various hardwood trees of the family Leguminosae
         [syn: locust tree, locust]