The Collaborative International Dictionary of English v.0.48:

Nephew \Neph"ew\ (n[e^]f"[-u]; in England n[e^]v"[-u]; 277), n.
   [OE. neveu, nevou, nevu, fr. F. neveu, OF. also, nevou, L.
   nepos; akin to AS. nefa, D. neef, G. neffe, OHG. nevo, Icel.
   nefi a kinsman, Gr. ne`podes, pl., brood, young, Skr.
   nep[=a]t grandson, descendant. [root]262. Cf. Niece,
   Nepotism.]
   1. A grandson or grandchild, or remoter lineal descendant.
      [Obs.]
      [1913 Webster]

            But if any widow have children or nephews [Rev. Ver.
            grandchildren].                     --1 Tim. v. 4.
      [1913 Webster]

            If naturalists say true that nephews are often liker
            to their grandfathers than to their fathers. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. A cousin. [Obs.] --Shak.
      [1913 Webster]

   3. The son of a brother or a sister, or of a brother-in-law
      or sister-in-law. --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nephew
    n 1: a son of your brother or sister [ant: niece]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "nephew":
   aunt, auntie, blood brother, brethren, brother, bub, bubba, bud,
   buddy, country cousin, cousin, cousin once removed,
   cousin twice removed, daughter, father, first cousin,
   foster brother, frater, grandnephew, grandniece, granduncle,
   great-aunt, great-uncle, half brother, kid brother, mother, niece,
   nuncle, nunks, nunky, second cousin, sis, sissy, sister,
   sister-german, sistern, son, stepbrother, stepsister, unc, uncle,
   uncs, uterine brother



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

NEPHEW, dom. rel. The son of a person's brother or sister. Amb. 514; 1
Jacob's Ch. R. 207.