The Collaborative International Dictionary of English v.0.48:

Mediocrity \Me`di*oc"ri*ty\, n. [F. m['e]diocrit['e], L.
   mediocritas.]
   [1913 Webster]
   1. The quality of being mediocre; a middle state or degree; a
      moderate degree or rate. "A mediocrity of success."
      --Bacon.
      [1913 Webster]

   2. Moderation; temperance. [Obs.] --Hooker.
      [1913 Webster]

   3. A mediocre person; -- used disparagingly.
      [PJC]

WordNet (r) 3.0 (2006):

mediocrity
    n 1: ordinariness as a consequence of being average and not
         outstanding [syn: averageness, mediocrity]
    2: a person of second-rate ability or value; "a team of aging
       second-raters"; "shone among the mediocrities who surrounded
       him" [syn: second-rater, mediocrity]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "mediocrity":
   Middle America, a nobody, a nothing, adulteration, average,
   balance, baseness, bourgeoisie, burgherdom, center, cipher,
   common man, defectibility, defectiveness, deficiency, dull tool,
   dummy, erroneousness, failure, fallibility, faultiness, fewness,
   figurehead, fourth-rater, generality, golden mean, greenhorn,
   happy medium, immaturity, impairment, imperfection, impurity,
   inability, inaccuracy, inadeptness, inadequacy, inadequateness,
   inaptitude, inaptness, inattentiveness, incapability, incapable,
   incapacity, incompetence, incompetency, incompetent,
   incompleteness, indexterity, ineffectiveness, ineffectuality,
   inefficiency, ineptness, inexactitude, inexactness, insignificancy,
   insufficiency, jackstraw, juste-milieu, lack, lightweight,
   little fellow, little guy, littleness, maladjustment,
   maladroitness, man of straw, mean, meanness, median, medium,
   middle, middle class, middle course, middle ground, middle order,
   middle orders, middle point, middle position, middle state,
   middle-of-the-road, midpoint, nebbish, no conjuror, no prize,
   nobody one knows, nobody special, nonentity, norm, normal,
   nothing special, obscurity, par, patchiness, pedestrianism,
   pettiness, pip-squeak, punk, rule, run, runt, rustiness, scrub,
   second-rater, shabbiness, shortage, shortcoming, shrimp,
   silent majority, sketchiness, skill-lessness, small beer,
   small fry, small potato, small potatoes, smallness, squirt, squit,
   subnormality, suburbia, third-rater, thoughtlessness, tinhorn,
   triviality, unaptness, uncleverness, undeftness, undevelopment,
   undexterousness, undextrousness, unevenness, unfitness,
   unfittedness, unintelligence, unperfectedness, unproficiency,
   unschooledness, unskillfulness, unsoundness, untrainedness,
   via media, vulgarity, want, whiffet, whippersnapper