The Collaborative International Dictionary of English v.0.48:

Eleventh \E*lev"enth\, a. [Cf. AS. endlyfta. See Eleven.]
   1. Next after the tenth; as, the eleventh chapter.
      [1913 Webster]

   2. Constituting one of eleven parts into which a thing is
      divided; as, the eleventh part of a thing.
      [1913 Webster]

   3. (Mus.) Of or pertaining to the interval of the octave and
      the fourth.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eleventh \E*lev"enth\, n.
   1. The quotient of a unit divided by eleven; one of eleven
      equal parts.
      [1913 Webster]

   2. (Mus.) The interval consisting of ten conjunct degrees;
      the interval made up of an octave and a fourth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eleventh
    adj 1: coming next after the tenth and just before the twelfth
           in position [syn: eleventh, 11th]
    n 1: position 11 in a countable series of things