An insurance company or party that shares in providing insurance coverage; one of multiple insurers covering the same risk.
From coinsure + -er (agent noun suffix). Emerged in insurance industry terminology for companies working together on large-scale risks.
If Lloyd's of London and AIG are both insuring the same mega-yacht, they're both coinsurers—they've divvied up the risk so if it sinks, one company doesn't eat the whole loss.
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