The Collaborative International Dictionary of English v.0.48:

Namesake \Name"sake`\, n. [For name's sake; i. e., one named for
   the sake of another's name.]
   One that has the same name as another; especially, one called
   after, or named out of regard to, another.
   [1913 Webster]

WordNet (r) 3.0 (2006):

namesake
    n 1: a person with the same name as another