The Collaborative International Dictionary of English v.0.48:

Boredom \Bore"dom\, n.
   1. The state of being bored, or pestered; a state of ennui.
      --Dickens.
      [1913 Webster]

   2. The realm of bores; bores, collectively.
      [1913 Webster]

WordNet (r) 3.0 (2006):

boredom
    n 1: the feeling of being bored by something tedious [syn:
         boredom, ennui, tedium]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "boredom":
   acedia, aloofness, angst, anguish, anxiety, apathy, boredness,
   carelessness, cheerlessness, detachment, discomfort, discomposure,
   discontent, disgust, disinterest, disinterestedness, dislike,
   dispiritedness, displeasure, disquiet, dissatisfaction, distaste,
   doldrums, dread, dreariness, drowsiness, dullness, emptiness,
   enervation, ennui, existential woe, fatigue, fed-upness, flatness,
   grimness, heaviness, hebetude, heedlessness, impassiveness,
   impassivity, inanimation, incuriosity, incuriousness, indifference,
   indifferentism, indifferentness, inquietude, insouciance,
   intellectual inertia, jadedness, joylessness, lack of interest,
   lack of pleasure, lackadaisicalness, languidness, languishment,
   languor, languorousness, lassitude, lenitude, lentor, lethargy,
   life-weariness, lifelessness, listlessness, malaise, melancholy,
   monotony, nausea, nongratification, nonsatisfaction, oscitancy,
   painfulness, passivity, phlegm, regardlessness, satedness,
   satiation, satiety, savorlessness, sleepiness, sloth, slothfulness,
   slowness, sluggishness, somnolence, spleen, staleness, stolidity,
   stupefaction, stupor, supineness, taedium vitae, tastelessness,
   tediousness, tedium, tiredness, torpidity, torpidness, torpitude,
   torpor, uncomfortableness, unconcern, unease, uneasiness,
   unhappiness, uninquisitiveness, uninterestedness, unmindfulness,
   unpleasure, unsatisfaction, vexation of spirit, wearifulness,
   weariness, withdrawal, world-weariness, yawn