NOUN (3)

1. strange and unconventional behavior;

2. (geometry) a ratio describing the shape of a conic section; the ratio of the distance between the foci to the length of the major axis;
- Example: "a circle is an ellipse with zero eccentricity"

3. a circularity that has a different center or deviates from a circular path;

The Collaborative International Dictionary of English v.0.48:

Eccentricity \Ec`cen*tric"i*ty\, n.; pl. Eccentricities. [Cf.
   F. excentricit['e].]
   1. The state of being eccentric; deviation from the customary
      line of conduct; oddity.
      [1913 Webster]

   2. (Math.) The ratio of the distance between the center and
      the focus of an ellipse or hyperbola to its
      semi-transverse axis.
      [1913 Webster]

   3. (Astron.) The ratio of the distance of the center of the
      orbit of a heavenly body from the center of the body round
      which it revolves to the semi-transverse axis of the
      orbit.
      [1913 Webster]

   4. (Mech.) The distance of the center of figure of a body, as
      of an eccentric, from an axis about which it turns; the
      throw.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eccentricity
    n 1: strange and unconventional behavior
    2: (geometry) a ratio describing the shape of a conic section;
       the ratio of the distance between the foci to the length of
       the major axis; "a circle is an ellipse with zero
       eccentricity"
    3: a circularity that has a different center or deviates from a
       circular path [ant: concentricity]

Moby Thesaurus II by Grady Ward, 1.0:

183 Moby Thesaurus words for "eccentricity":
   aberrance, aberrancy, aberration, abnormality, abnormity,
   absurdity, absurdness, amorphism, animus, anomalism, anomalousness,
   anomaly, aptitude, arrhythmia, asininity, battiness, bent, bias,
   bizarreness, bizarrerie, brainlessness, brokenness, buffoonery,
   capriciousness, cast, character, choppiness, clownishness,
   constitution, crackpottedness, crankiness, craziness, daffiness,
   derangement, desipience, desultoriness, deviability, deviation,
   diathesis, difference, disconnectedness, discontinuity,
   disposition, divergence, drollery, drollness, erraticism,
   erraticness, fatuity, fatuousness, fibrillation, fickleness,
   fitfulness, fits and starts, flightiness, fluctuation, folly,
   foolery, foolheadedness, foolishness, freakishness, frivolity,
   frivolousness, funniness, giddiness, goofiness, grain,
   heteromorphism, hilarity, humorousness, idiocy, idiosyncrasy,
   imbecility, impulsiveness, inanity, inclination, incongruity,
   inconsistency, inconstancy, individualism, ineptitude, inferiority,
   insanity, instability, intermittence, irregularity, jerkiness,
   kidney, laughability, leaning, ludicrousness, lunacy, madness,
   make, makeup, mental set, mercuriality, mettle, mind, mind-set,
   mindlessness, mold, monstrosity, moodiness, nature, niaiserie,
   nonuniformity, nugacity, nuttiness, patchiness, predilection,
   predisposition, preference, pricelessness, proclivity, propensity,
   quaintness, queerness, quizzicalness, restlessness, richness,
   ridiculousness, roughness, sappiness, screwiness, senselessness,
   set, shiftiness, silliness, slant, spasticity, sporadicity,
   sporadicness, spottiness, stagger, stamp, strain, streak, stripe,
   stupidity, subnormality, superiority, temper, temperament,
   tendency, teratism, the funny side, thoughtlessness, triflingness,
   triviality, turn, turn of mind, twist, type, uncertainty,
   undependability, unevenness, unfixedness, unmethodicalness,
   unnaturalism, unnaturalness, unpredictability, unreliability,
   unsettledness, unstableness, unsteadfastness, unsteadiness,
   unsystematicness, variability, variation, variety, wackiness,
   wantonness, warp, waywardness, weirdness, whimsicality,
   whimsicalness, witlessness, wittiness, wobble, zaniness, zanyism



The Devil's Dictionary (1881-1906):

ECCENTRICITY, n.  A method of distinction so cheap that fools employ
it to accentuate their incapacity.