Source : Webster's Revised Unabridged Dictionary (1913)

Rapid \Rap"id\, a. [L. rapidus, fr. rapere to seize and carry
   off, to snatch or hurry away; perhaps akin to Gr. ?; cf. F.
   rapide. Cf. {Harpy}, {Ravish}.]
   1. Very swift or quick; moving with celerity; fast; as, a
      rapid stream; a rapid flight; a rapid motion.

            Ascend my chariot; guide the rapid wheels. --Milton.

   2. Advancing with haste or speed; speedy in progression; in
      quick sequence; as, rapid growth; rapid improvement; rapid
      recurrence; rapid succession.

   3. Quick in execution; as, a rapid penman.

Rapid \Rap"id\, n. [Cf. F. rapide. See {Rapid}, a.]
   The part of a river where the current moves with great
   swiftness, but without actual waterfall or cascade; --
   usually in the plural; as, the Lachine rapids in the St.
   Lawrence.

Source : WordNet®

rapid
     adj 1: done or occurring in a brief period of time; "a rapid rise
            through the ranks"
     2: characterized by speed; moving with or capable of moving
        with high speed; "a rapid movment"; "a speedy car"; "a
        speedy errand boy" [syn: {speedy}]

rapid
     n : a part of a river where the current is very fast