The Collaborative International Dictionary of English v.0.48:

Coinsurance \Co`in*sur"ance\, n. [Co- + insurance.]
   Insurance jointly with another or others; specif., that
   system of fire insurance in which the insurer is treated as
   insuring himself to the extent of that part of the risk not
   covered by his policy, so that any loss is apportioned
   between him and the insurance company on the principle of
   average, as in marine insurance or between other insurers.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

coinsurance
    n 1: insurance issued jointly by two or more underwriters