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]

The Collaborative International Dictionary of English v.0.48:

Acacia \A*ca"ci*a\, n. (Antiq.)
   A roll or bag, filled with dust, borne by Byzantine emperors,
   as a memento of mortality. It is represented on medals.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

acacia \a*ca"cia\ ([.a]*k[=a]"sh[.a] or [.a]*k[=a]"sh[i^]*[.a]),
   n.; pl. E. acacias ([.a]*k[=a]"sh[.a]z), L. acaciae
   ([.a]*k[=a]"sh[i^]*[=e]). [L. from Gr. 'akaki`a; orig. the
   name of a thorny tree found in Egypt; prob. fr. the root ak
   to be sharp. See Acute.]
   1. [capitalized] A genus of leguminous trees and shrubs.
      Nearly 300 species are Australian or Polynesian, and have
      terete or vertically compressed leaf stalks, instead of
      the bipinnate leaves of the much fewer species of America,
      Africa, etc. Very few are found in temperate climates.
      [1913 Webster]

   2. (Med.) The inspissated juice of several species of acacia;
      -- called also gum acacia, and gum arabic. AS
      [1913 Webster] Acacin

WordNet (r) 3.0 (2006):

acacia
    n 1: any of various spiny trees or shrubs of the genus Acacia

Easton's 1897 Bible Dictionary:

Acacia
   (Heb. shittim) Ex. 25:5, R.V. probably the Acacia seyal (the
   gum-arabic tree); called the "shittah" tree (Isa. 41:19). Its
   wood is called shittim wood (Ex. 26:15,26; 25:10,13,23,28,
   etc.). This species (A. seyal) is like the hawthorn, a gnarled
   and thorny tree. It yields the gum-arabic of commerce. It is
   found in abundance in the Sinaitic peninsula.