The Collaborative International Dictionary of English v.0.48:

Grandfather \Grand"fa"ther\, n.
   A father's or mother's father; an ancestor immediately after
   the father or mother in lineal ascent.
   [1913 Webster]

   Grandfather longlegs. (Zool.) See Daddy longlegs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grandfather
    n 1: the father of your father or mother [syn: grandfather,
         gramps, granddad, grandad, granddaddy, grandpa]