The Collaborative International Dictionary of English v.0.48:

Opposition \Op`po*si"tion\, n. [F., fr. L. oppositio. See
   Opposite.]
   1. The act of opposing; an attempt to check, restrain, or
      defeat; resistance.
      [1913 Webster]

            The counterpoise of so great an opposition. --Shak.
      [1913 Webster]

            Virtue which breaks through all opposition.
                                                  --Milton.
      [1913 Webster]

   2. The state of being placed over against; situation so as to
      front something else. --Milton.
      [1913 Webster]

   3. Repugnance; contrariety of sentiment, interest, or
      purpose; antipathy. --Shak.
      [1913 Webster]

   4. That which opposes; an obstacle; specifically, the
      aggregate of persons or things opposing; hence, in
      politics and parliamentary practice, the party opposed to
      the party in power.
      [1913 Webster]

   5. (Astron.) The situation of a heavenly body with respect to
      another when in the part of the heavens directly opposite
      to it; especially, the position of a planet or satellite
      when its longitude differs from that of the sun 180[deg];
      -- signified by the symbol ?; as, ? [Jupiter] [Sun],
      opposition of Jupiter to the sun.
      [1913 Webster]

   6. (Logic) The relation between two propositions when, having
      the same subject and predicate, they differ in quantity,
      or in quality, or in both; or between two propositions
      which have the same matter but a different form.
      [1913 Webster]

WordNet (r) 3.0 (2006):

opposition
    n 1: the action of opposing something that you disapprove or
         disagree with; "he encountered a general feeling of
         resistance from many citizens"; "despite opposition from
         the newspapers he went ahead" [syn: resistance,
         opposition]
    2: the relation between opposed entities [syn: opposition,
       oppositeness]
    3: the act of hostile groups opposing each other; "the
       government was not ready for a confrontation with the
       unions"; "the invaders encountered stiff opposition" [syn:
       confrontation, opposition]
    4: a contestant that you are matched against [syn: opposition,
       opponent, opposite]
    5: a body of people united in opposing something
    6: a direction opposite to another
    7: an armed adversary (especially a member of an opposing
       military force); "a soldier must be prepared to kill his
       enemies" [syn: enemy, foe, foeman, opposition]
    8: the major political party opposed to the party in office and
       prepared to replace it if elected; "Her Majesty's loyal
       opposition"

Moby Thesaurus II by Grady Ward, 1.0:

298 Moby Thesaurus words for "opposition":
   adversary, adversity, agreement to disagree, alienation, allegory,
   analogy, antagonism, antagonist, antagonistic, anteposition,
   antipathy, antithesis, antithetical, apostasy, argumentation,
   arrest, arrestation, arrestment, assailant, at daggers drawn,
   averseness, aversion, backlash, backwardness, balancing, ban,
   blackball, blackballing, blockage, blocking, challenge, check,
   clashing, clogging, closing up, closure, collision, combatant,
   combative reaction, comparative anatomy, comparative degree,
   comparative grammar, comparative judgment, comparative linguistics,
   comparative literature, comparative method, compare, comparing,
   comparison, competing, competition, competitive, competitor,
   complaint, con, conflict, conflicting, confrontation, confrontment,
   confutation, constriction, contention, contradiction,
   contradistinction, contraindication, contraposition, contrariety,
   contrast, contrastiveness, controversy, correlation,
   counter-culture, counteraction, counterposition, counterworking,
   cramp, crankiness, cross-purposes, crotchetiness, cursoriness,
   defiance, delay, demur, departure, detainment, detention,
   deviation, difference, dim view, disaccord, disaccordance,
   disagreement, disappointment, disapprobation, disapproval,
   disconformity, discongruity, discontent, discontentedness,
   discontentment, discord, discordance, discordancy, discrepancy,
   discreteness, disenchantment, disesteem, disfavor, disgruntlement,
   disharmony, disillusion, disillusionment, disinclination,
   disobedience, disparity, displeasure, dispute, disrelish,
   disrespect, dissatisfaction, dissension, dissent, dissentience,
   dissidence, dissimilarity, dissonance, distaste, distinction,
   distinctiveness, distinctness, disunion, disunity, divergence,
   divergency, diversity, dropping out, enemy, exclusion, faction,
   far cry, fixation, flak, foe, foeman, foot-dragging, fractiousness,
   friction, grudging consent, grudgingness, hampering, heterogeneity,
   hindering, hindrance, holdback, holdup, hostile, hostility,
   impediment, in opposition, inaccordance, incompatibility,
   incongruity, inconsistency, inconsonance, indignation,
   indisposedness, indisposition, indocility, inequality,
   inharmoniousness, inharmony, inhibition, inimicalness,
   interference, interruption, intractableness, irreconcilability,
   jarring, kick, lack of enthusiasm, lack of zeal, let, likening,
   low estimation, low opinion, matching, metaphor, minority opinion,
   mixture, mutinousness, negation, negativism, nolition,
   nonagreement, nonassent, nonconcurrence, nonconformity, nonconsent,
   noncooperation, nuisance value, objection, obstinacy, obstruction,
   obstructionism, occlusion, odds, opponent, opposed, opposing,
   opposing party, opposite camp, oppositeness, opposure, oppugnance,
   oppugnancy, ostracism, other side, otherness, parallelism,
   passive resistance, perfunctoriness, perverseness, perversity,
   polar opposition, polarity, polarization, posing against,
   proportion, protest, reaction, rebuff, recalcitrance,
   recalcitrancy, recalcitration, recoil, recusance, recusancy,
   refractoriness, refusal, rejection, relation, reluctance,
   renitence, renitency, repellence, repellency, repercussion,
   repression, repudiation, repugnance, repulse, repulsion,
   resistance, restraint, restriction, retardation, retardment,
   revolt, rival, secession, separateness, setback, showdown, simile,
   similitude, slowness, squeeze, stand, stranglehold, stricture,
   stubbornness, sulk, sulkiness, sulks, sullenness, suppression,
   swimming upstream, the loyal opposition, the opposition,
   thumbs-down, trope of comparison, unconformity, uncooperativeness,
   underground, unenthusiasm, unfriendliness, unhappiness,
   unharmoniousness, unlikeness, unorthodoxy, unwillingness, variance,
   variation, variegation, variety, weighing, withdrawal,
   withstanding



Bouvier's Law Dictionary, Revised 6th Ed (1856):

OPPOSITION, practice. The act of a creditor who, declares his dissent to a
debtor's being discharged under the insolvent laws.



The Devil's Dictionary (1881-1906):

OPPOSITION, n.  In politics the party that prevents the Government from
running amuck by hamstringing it.
    The King of Ghargaroo, who had been abroad to study the science of
government, appointed one hundred of his fattest subjects as members
of a parliament to make laws for the collection of revenue.  Forty of
these he named the Party of Opposition and had his Prime Minister
carefully instruct them in their duty of opposing every royal measure.
Nevertheless, the first one that was submitted passed unanimously.
Greatly displeased, the King vetoed it, informing the Opposition that
if they did that again they would pay for their obstinacy with their
heads.  The entire forty promptly disemboweled themselves.
    "What shall we do now?" the King asked.  "Liberal institutions
cannot be maintained without a party of Opposition."
    "Splendor of the universe," replied the Prime Minister, "it is
true these dogs of darkness have no longer their credentials, but all
is not lost.  Leave the matter to this worm of the dust."
    So the Minister had the bodies of his Majesty's Opposition
embalmed and stuffed with straw, put back into the seats of power and
nailed there.  Forty votes were recorded against every bill and the
nation prospered.  But one day a bill imposing a tax on warts was
defeated -- the members of the Government party had not been nailed to
their seats!  This so enraged the King that the Prime Minister was put
to death, the parliament was dissolved with a battery of artillery,
and government of the people, by the people, for the people perished
from Ghargaroo.