Source : Webster's Revised Unabridged Dictionary (1913)

Niece \Niece\, n. [OE. nece, F. ni[`e]ce, LL. neptia, for L.
   neptis a granddaughter, niece, akin to nepos. See {Nephew}.]
   1. A relative, in general; especially, a descendant, whether
      male or female; a granddaughter or a grandson. [Obs.] --B.
      Jonson. Wyclif. Shak.

   2. A daughter of one's brother or sister, or of one's
      brother-in-law or sister-in-law.

Source : WordNet®

niece
     n : a daughter of your brother or sister [ant: {nephew}]