The outstanding claims reserve is the provision made in the balance sheet of an insurance company for all claims that have been made and for which the insurer is liable, but which had not been settled at the balance sheet date.
It may also be used more broadly to mean all liabilities incurred but not settled. In this case it include liabilities incurred (i.e. an insured event happened) but not yet reported (to the insurer) which would otherwise be shown in a separate IBNR reserve as well as claims that may be reopened.
The outstanding claims reserve is an actuarial estimate, as the amounts liable on any given claim is not know until settlement.