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):

cisco
    n 1: cold-water fish caught in Lake Superior and northward [syn:
         lake herring, cisco]
    2: important food fish of cold deep lakes of North America [syn:
       cisco, lake herring, Coregonus artedi]