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.
      [1913 Webster]

   3. Easy; unobstructed. [Obs.]
      [1913 Webster]

            The reception of the light into the body of the
            building was very prompt.             --Sir H.
                                                  Wotton.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "prompter":
   MC, abettor, actuator, aide-memoire, animator, auteur,
   ballyhoo man, barker, cajoler, callboy, coax, coaxer,
   costume designer, costumer, costumier, cue, director, emcee,
   encourager, energizer, equestrian director, exhibitor, firer,
   flapper, gadfly, galvanizer, hint, impeller, impresario, inducer,
   inspirer, jogger, makeup man, master of ceremonies, memorandum,
   mover, moving spirit, persuader, playreader, pleader, prime mover,
   producer, prompt, prompting, remembrance, remembrancer, reminder,
   ringmaster, scenewright, set designer, showman, spark, spark plug,
   sparker, spieler, stage director, stage manager, stimulator,
   tempter, theater man, theatrician, ticket collector, tickler,
   usher, usherer, usherette, wheedler