The Collaborative International Dictionary of English v.0.48:

Prompt \Prompt\ (pr[o^]mt; 215), a. [Compar. Prompter; superl.
   Promptest.] [F. prompt, L. promptus, properly, brought
   forth (to light or view), hence, visible, evident, at hand,
   ready, quick, -- p. p. of promere to take or bring forth; pro
   forth + emere to take. See Redeem. ]
   [1913 Webster]
   1. Ready and quick to act as occasion demands; meeting
      requirements readily; not slow, dilatory, or hesitating in
      decision or action; responding on the instant; immediate;
      as, prompt in obedience or compliance; -- said of persons.
      [1913 Webster]

            Very discerning and prompt in giving orders.
                                                  --Clarendon.
      [1913 Webster]

            Tell him I am prompt
            To lay my crown at's feet.            --Shak.
      [1913 Webster]

            And you, perhaps, too prompt in your replies.
                                                  --Dryden.
      [1913 Webster]

   2. Done or rendered quickly, readily, or immediately; given
      without delay or hesitation; -- said of conduct; as,
      prompt assistance.
      [1913 Webster]

            When Washington heard the voice of his country in
            distress,
            his obedience was prompt.             --Ames.
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   3. Easy; unobstructed. [Obs.]
      [1913 Webster]

            The reception of the light into the body of the
            building was very prompt.             --Sir H.
                                                  Wotton.
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   Syn: Ready; expeditious; quick; agile; alert; brisk; nimble.

   Usage: Prompt, Ready, Expeditious. One who is ready is
          prepared to act at the moment. One who is prompt acts
          at the moment. One who is expeditious carries through
          an undertaking with constant promptness.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Prompter \Prompt"er\, n.
   [1913 Webster]
   1. One who, or that which, prompts; one who admonishes or
      incites to action.
      [1913 Webster]

   2. One who reminds another, as an actor or an orator, of the
      words to be spoken next; specifically, one employed for
      this purpose in a theater.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prompter
    n 1: someone who assists a performer by providing the next words
         of a forgotten speech [syn: prompter, theater prompter]
    2: a device that displays words for people to read [syn:
       prompter, autocue]