The Collaborative International Dictionary of English v.0.48:

Cisco \Cis"co\, n. (Zool.)
   The Lake herring (Coregonus Artedi), valuable food fish of
   the Great Lakes of North America. The name is also applied to
   Coregonus Hoyi, a related species of Lake Michigan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Coregonus artedi
    n 1: important food fish of cold deep lakes of North America
         [syn: cisco, lake herring, Coregonus artedi]