The Collaborative International Dictionary of English v.0.48:

Stopper \Stop"per\, n.
   1. One who stops, closes, shuts, or hinders; that which stops
      or obstructs; that which closes or fills a vent or hole in
      a vessel.
      [1913 Webster]

   2. (Naut.) A short piece of rope having a knot at one or both
      ends, with a lanyard under the knot, -- used to secure
      something. --Totten.
      [1913 Webster]

   3. (Bot.) A name to several trees of the genus Eugenia, found
      in Florida and the West Indies; as, the red stopper. See
      Eugenia. --C. S. Sargent.
      [1913 Webster]

   Ring stopper (Naut.), a short rope or chain passing through
      the anchor ring, to secure the anchor to the cathead.

   Stopper bolt (Naut.), a large ringbolt in a ship's deck, to
      which the deck stoppers are hooked.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Stopper \Stop"per\, v. t. [imp. & p. p. Stoppered; p. pr. &
   vb. n. Stoppering.]
   To close or secure with a stopper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stopper
    n 1: an act so striking or impressive that the show must be
         delayed until the audience quiets down [syn: show-
         stopper, showstopper, stopper]
    2: a remark to which there is no polite conversational reply
       [syn: conversation stopper, stopper]
    3: (bridge) a playing card with a value sufficiently high to
       insure taking a trick in a particular suit; "if my partner
       has a spade stopper I can bid no trump"
    4: blockage consisting of an object designed to fill a hole
       tightly [syn: plug, stopper, stopple]
    v 1: close or secure with or as if with a stopper; "She
         stoppered the wine bottle"; "The mothers stoppered their
         babies' mouths with pacifiers" [syn: stopper, stopple]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "stopper":
   KO, bar, bind, block, block up, blockade, bonnet, bung, cap, caulk,
   ceil, check valve, chink, choke, choke off, choke up, clincher,
   clog, clog up, close, cock, coif, congest, constipate, cork, cover,
   crown, crusher, dam, dam up, death stroke, deathblow, dome,
   end-all, ender, faucet, fill, fill up, final stroke, finisher,
   finishing stroke, foul, hat, hood, jam, kayo, kayo punch, knockout,
   knockout blow, last dab, lid, obstipate, obstruct, occlude,
   operculum, pack, peg, pin, plug, plug up, quietus, roof, roof in,
   sea cock, settler, sockdolager, spigot, spike, spile, spill,
   stanch, stay, stench, stop, stop up, stopgap, stopple, stuff,
   stuff up, tap, tip, top, valve