The Collaborative International Dictionary of English v.0.48:

Fifteenth \Fif"teenth`\, a. [OE. fiftenthe; cf. fiftethe, AS.
   f[imac]fte[=o][eth]a. See Fifteen.]
   1. Next in order after the fourteenth; -- the ordinal of
      fifteen.
      [1913 Webster]

   2. Consisting of one of fifteen equal parts or divisions of a
      thing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fifteenth \Fif"teenth`\, n.
   1. One of fifteen equal parts or divisions; the quotient of a
      unit divided by fifteen.
      [1913 Webster]

   2. A species of tax upon personal property formerly laid on
      towns, boroughs, etc., in England, being one fifteenth
      part of what the personal property in each town, etc., had
      been valued at. --Burrill.
      [1913 Webster]

   3. (Mus.)
      (a) A stop in an organ tuned two octaves above the
          diaposon.
      (b) An interval consisting of two octaves.
          [1913 Webster]

WordNet (r) 3.0 (2006):

fifteenth
    adj 1: coming next after the fourteenth and just before the
           sixteenth in position [syn: fifteenth, 15th]
    n 1: position 15 in a countable series of things

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FIFTEENTH, Eng. law. The name of a tax levied by authority of parliament for
the use of the king, which consisted of one-fifteenth part of the goods of
those who are subject to it. T. L