The Collaborative International Dictionary of English v.0.48:

Co-respondent \Co`-re*spond"ent\ (k?`rr?-sp?nd"ent), n. (Law)
   One who is called upon to answer a summons or other
   proceeding jointly with another.
   [1913 Webster]

WordNet (r) 3.0 (2006):

corespondent
    n 1: the codefendant charged with adultery with the estranged
         spouse in a divorce proceeding [syn: corespondent, co-
         respondent]