The Collaborative International Dictionary of English v.0.48:

Tedium \Te"di*um\, n. [L. taedium, fr. taedet it disgusts, it
   wearies one.]
   Irksomeness; wearisomeness; tediousness. [Written also
   taedium.] --Cowper.
   [1913 Webster]

         To relieve the tedium, he kept plying them with all
         manner of bams.                          --Prof.
                                                  Wilson.
   [1913 Webster]

         The tedium of his office reminded him more strongly of
         the willing scholar, and his thoughts were rambling.
                                                  --Dickens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tedium
    n 1: the feeling of being bored by something tedious [syn:
         boredom, ennui, tedium]
    2: dullness owing to length or slowness [syn: tediousness,
       tedium, tiresomeness]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "tedium":
   alliteration, angst, anguish, anxiety, assonance, banality,
   boredom, changelessness, cheerlessness, chime, colorlessness,
   dingdong, discomfort, discomposure, discontent, dislike,
   displeasure, disquiet, dissatisfaction, doldrums, drabness, dread,
   dreariness, drone, dullness, emptiness, ennui, existential woe,
   flatness, grimness, harping, humdrum, inquietude, insipidity,
   insipidness, invariability, irksomeness, jingle, jingle-jangle,
   joylessness, lack of pleasure, long-windedness, malaise, monotone,
   monotony, nausea, near rhyme, nongratification, nonsatisfaction,
   painfulness, pitter-patter, repeated sounds, repetitiousness,
   repetitiveness, rhyme, routine, savorlessness, singsong,
   slant rhyme, spleen, stale repetition, staleness, tastelessness,
   tediousness, tiresomeness, trot, uncomfortableness, unease,
   uneasiness, unhappiness, unnecessary repetition, unpleasure,
   unsatisfaction, vapidity, vexation of spirit, wearisomeness,
   yawn



The Devil's Dictionary (1881-1906):

TEDIUM, n.  Ennui, the state or condition of one that is bored.  Many
fanciful derivations of the word have been affirmed, but so high an
authority as Father Jape says that it comes from a very obvious
source -- the first words of the ancient Latin hymn _Te Deum
Laudamus_.  In this apparently natural derivation there is something
that saddens.