The Collaborative International Dictionary of English v.0.48:

Locust \Lo"cust\, n. [L. locusta locust, grasshopper. Cf.
   Lobster.]
   1. (Zool.) Any one of numerous species of long-winged,
      migratory, orthopterous insects, of the family
      Acridid[ae], allied to the grasshoppers; esp., (Edipoda
      migratoria, syn. Pachytylus migratoria, and Acridium
      perigrinum, of Southern Europe, Asia, and Africa. In the
      United States the related species with similar habits are
      usually called grasshoppers. See Grasshopper.
      [1913 Webster]

   Note: These insects are at times so numerous in Africa and
         the south of Asia as to devour every green thing; and
         when they migrate, they fly in an immense cloud. In the
         United States the harvest flies are improperly called
         locusts. See Cicada.
         [1913 Webster]

   Locust beetle (Zool.), a longicorn beetle (Cyllene
      robini[ae]), which, in the larval state, bores holes in
      the wood of the locust tree. Its color is brownish black,
      barred with yellow. Called also locust borer.

   Locust bird (Zool.) the rose-colored starling or pastor of
      India. See Pastor.

   Locust hunter (Zool.), an African bird; the beefeater.
      [1913 Webster]

   2. [Etymol. uncertain.] (Bot.) The locust tree. See Locust
      Tree (definition, note, and phrases).
      [1913 Webster]

   Locust bean (Bot.), a commercial name for the sweet pod of
      the carob tree.
      [1913 Webster]