The Collaborative International Dictionary of English v.0.48:

By-election \By"-e*lec"tion\, n.
   An election held by itself, not at the time of a general
   election.
   [1913 Webster]

WordNet (r) 3.0 (2006):

by-election
    n 1: a special election between regular elections [syn: by-
         election, bye-election]