From The Collaborative International Dictionary of English v.0.48:

Locust tree Lo"cust tree` . [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]

From 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]

From 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

From WordNet (r) 2.0:

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

From 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.