The Collaborative International Dictionary of English v.0.48:

Objection \Ob*jec"tion\, n. [L. objectio: cf. F. objection.]
   1. The act of objecting; as, to prevent agreement, or action,
      by objection. --Johnson.
      [1913 Webster]

   2. That which is, or may be, presented in opposition; an
      adverse reason or argument; a reason for objecting;
      obstacle; impediment; as, I have no objection to going;
      unreasonable objections. "Objections against every truth."
      --Tyndale.
      [1913 Webster]

   3. Cause of trouble; sorrow. [Obs. or R.]
      [1913 Webster]

            He remembers the objection that lies in his bosom,
            and he sighs deeply.                  --Jer. Taylor.
      [1913 Webster]

   Syn: Exception; difficulty; doubt; scruple.
        [1913 Webster]

WordNet (r) 3.0 (2006):

objection
    n 1: the act of expressing earnest opposition or protest [syn:
         expostulation, remonstrance, remonstration,
         objection]
    2: the speech act of objecting
    3: the act of protesting; a public (often organized)
       manifestation of dissent [syn: protest, objection,
       dissent]
    4: (law) a procedure whereby a party to a suit says that a
       particular line of questioning or a particular witness or a
       piece of evidence or other matter is improper and should not
       be continued and asks the court to rule on its impropriety or
       illegality

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "objection":
   answer, antipathy, argument, ban, bashfulness, beef, bitch,
   blackball, blackballing, block, blockade, boggle, boggling,
   bottleneck, boycott, catch, challenge, combative reaction,
   complaint, compunction, cordon, counteraction, counterstatement,
   curtain, defense, defiance, demonstration, demur, demurral,
   demurrer, denial, determent, deterrent, difficulty, diffidence,
   dim view, disagreement, disappointment, disapprobation,
   disapproval, discontent, discontentedness, discontentment,
   disenchantment, disesteem, disfavor, disgruntlement, disillusion,
   disillusionment, dislike, displeasure, dispute, disrespect,
   dissatisfaction, dissent, dissentience, distaste, doubt, drawback,
   exception, exclusion, expostulation, falter, faltering,
   fractiousness, grievance, grievance committee, hang-up, hazard,
   hesitance, hesitancy, hesitation, hitch, howl, hurdle, indignation,
   indignation meeting, interference, joker, kick, low estimation,
   low opinion, march, modesty, negativism, noncooperation,
   nonviolent protest, obstacle, obstinacy, obstruction, obstructive,
   one small difficulty, opposition, opposure, ostracism,
   passive resistance, pause, picketing, plea, pleading, protest,
   protest demonstration, protestation, qualm, qualm of conscience,
   qualmishness, question, rally, reaction, rebuff, rebuttal,
   recalcitrance, recalcitrancy, recalcitration, recoil,
   refractoriness, refusal, refutation, rejection, reluctance,
   remonstrance, remonstration, renitence, renitency, repellence,
   repellency, reply, repulse, repulsion, resistance, response,
   revolt, riposte, rub, scruple, scrupulosity, scrupulousness,
   shrinking, shyness, sit-in, snag, special demurrer,
   special pleading, squawk, stand, statement of defense, stickling,
   strike, stumbling block, stumbling stone, teach-in, thumbs-down,
   uncooperativeness, unhappiness, withstanding