The Collaborative International Dictionary of English v.0.48:

Overturn \O`ver*turn"\, v. t. [imp. & p. p. Overturned; p. pr.
   & vb. n. Overturning.]
   1. To turn or throw from a basis, foundation, or position; to
      overset; as, to overturn a carriage or a building.
      [1913 Webster]

   2. To subvert; to destroy; to overthrow.
      [1913 Webster]

   3. To overpower; to conquer. --Milton.
      [1913 Webster]

   Syn: To demolish; overthrow. See Demolish.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overturn \O"ver*turn`\, n.
   The act off overturning, or the state of being overturned or
   subverted; overthrow; as, an overturn of parties.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overturn
    n 1: the act of upsetting something; "he was badly bruised by
         the upset of his sled at a high speed" [syn: upset,
         overturn, turnover]
    2: an improbable and unexpected victory; "the biggest upset
       since David beat Goliath" [syn: overturn, upset]
    v 1: turn from an upright or normal position; "The big vase
         overturned"; "The canoe tumped over" [syn: overturn,
         turn over, tip over, tump over]
    2: cause to overturn from an upright or normal position; "The
       cat knocked over the flower vase"; "the clumsy customer
       turned over the vase"; "he tumped over his beer" [syn:
       overturn, tip over, turn over, upset, knock over,
       bowl over, tump over]
    3: rule against; "The Republicans were overruled when the House
       voted on the bill" [syn: overrule, overturn, override,
       overthrow, reverse]
    4: cause the downfall of; of rulers; "The Czar was overthrown";
       "subvert the ruling class" [syn: overthrow, subvert,
       overturn, bring down]
    5: cancel officially; "He revoked the ban on smoking"; "lift an
       embargo"; "vacate a death sentence" [syn: revoke, annul,
       lift, countermand, reverse, repeal, overturn,
       rescind, vacate]
    6: change radically; "E-mail revolutionized communication in
       academe" [syn: revolutionize, revolutionise, overturn]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "overturn":
   Waterloo, answer, answer conclusively, argue down, be lost,
   beating, bloodless revolution, bouleversement, breakdown, breakup,
   bring down, capsizal, capsize, careen, cataclysm, catastrophe,
   clean slate, clean sweep, collapse, computer revolution, confound,
   confute, conquering, conquest, contradict, controvert, convulsion,
   counterrevolution, crash, crush, culbute, culbuter, deathblow,
   debacle, deck, defeat, demolish, deny, depose, destruction,
   dismiss, dispose of, down, downcast, downfall, downthrow, drubbing,
   eject, failure, fall, finish, floor, founder, go down, hiding,
   honeycomb, invert, keel, keel over, knock down, knock over,
   lambasting, lathering, licking, mastery, nonplus, oust, ouster,
   ousting, overcome, overcoming, overmaster, overmatch, overpower,
   overset, overthrow, overturning, overwhelm, palace revolution,
   parry, pitchpole, precipitation, prostrate, prostration,
   put to silence, quietus, radical change, rebut, reduce to silence,
   refute, reorganization, revolt, revolute, revolution,
   revolutionary war, revolutionize, revulsion, ruin, sabotage, sap,
   sap the foundations, scuttle, send flying, settle, shake-up,
   shut up, silence, sink, smash, smash all opposition, somersault,
   somerset, spasm, spill, squash, squelch, striking alteration,
   subdual, subduing, subjugation, subversion, subvert, surmount,
   sweeping change, tabula rasa, technological revolution, thrashing,
   throw down, throw over, tip over, topple, topple over, toppling,
   topsy-turvify, topsy-turvy, total change, transilience, trimming,
   trip, trip up, trouncing, tumble, turn a somersault, turn over,
   turn topsy-turvy, turn turtle, turn upside down, turnover,
   undermine, undoing, unhorse, unseat, unseating, unthrone, upend,
   upheaval, upset, upset the boat, upturn, vanquishment,
   violent change, weaken, whipping