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]

The Devil's Dictionary (1881-1906):

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