What is Co-insurance (coassurance)?

Several insurers jointly accepting one large risk in shares agreed in advance. Usually one of them is the leader: it agrees the terms, services the policy and handles the claim, while the others cover their own shares. Each insurer is liable only for its own share. The difference from reinsurance: in reinsurance the policyholder concludes a contract with one insurer, which itself passes part of the risk to another company; in co-insurance all the insurers share the risk from the outset. Be careful: the English word "coinsurance" is sometimes also used in another sense, meaning that the insured person pays part of each loss himself.

Source: Co-insurance (Wikipedia); HMRC Insurance Premium Tax Manual, IPT03530