The Collaborative International Dictionary of English v.0.48:

Rapid \Rap"id\ (r[a^]p"[i^]d), a. [L. rapidus, fr. rapere to
   seize and carry off, to snatch or hurry away; perhaps akin to
   Gr. 'arpa`zein: cf. F. rapide. Cf. Harpy, Ravish.]
   [1913 Webster]
   1. Very swift or quick; moving with celerity; fast; as, a
      rapid stream; a rapid flight; a rapid motion.
      [1913 Webster]

            Ascend my chariot; guide the rapid wheels. --Milton.
      [1913 Webster]

   2. Advancing with haste or speed; speedy in progression; in
      quick sequence; as, rapid growth; rapid improvement; rapid
      recurrence; rapid succession.
      [1913 Webster]

   3. Quick in execution; as, a rapid penman.
      [1913 Webster] Rapid

The Collaborative International Dictionary of English v.0.48:

Rapid \Rap"id\, Rapids \Rap"ids\(r[a^]p"[i^]dz), 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; sometimes
   called whitewater; -- usually used in the plural; as, the
   Lachine rapids in the St. Lawrence. For boaters on the river,
   it is a place that can be hazardous, with danger of capsizing
   or crashing into large rocks.
   [1913 Webster +PJC]

         Row, brothers, row, the stream runs fast,
         The rapids are near, and the daylight's past. --Moore.
   [1913 Webster] Rapid-fire

WordNet (r) 3.0 (2006):

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 movement"; "a speedy car"; "a speedy
       errand boy" [syn: rapid, speedy]
    n 1: a part of a river where the current is very fast