The Collaborative International Dictionary of English v.0.48:

Grandmother \Grand"moth"er\, n.
   The mother of one's father or mother.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grandmother
    n 1: the mother of your father or mother [syn: grandma,
         grandmother, granny, grannie, gran, nan, nanna]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "grandmother":
   beldam, crone, dame, dowager, frump, gammer, grandam, grandma,
   grannam, granny, great-grandmother, hag, old battle-ax, old dame,
   old girl, old granny, old lady, old trot, old wife, old woman,
   trot, war-horse, witch



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

GRANDMOTHER, domestic relations. The mother of one's father or mother. The
father's mother is called the paternal grandmother; the mother's mother is
the maternal grandmother.