Underinsurance

(i) A condition in which not enough Insurance is carried to cover the value of an exposure, (ii) Condition in which the amount of property Insurance is less than the applicable Coinsurance requirement, See Also: “Coinsurance.”
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A situation wherein the owner of a property or the person suffering a health condition does not have enough insurance to cover the value of the item or the health care costs.

Underinsured motorists coverage

Coverage for the insured and passengers whenever the at-fault driver in an accident has auto liability insurance with lesser limits than the insured’s. This coverage lies atop uninsured motorists coverage or atop the at-fault driver’s low limit automobile liability insurance and provides the insured and passengers with protection equal (usually) to the insured’s own automobile liability cover.

Underlying

The amount of insurance or reinsurance on a risk (or occurrence) that applies to a loss before the next higher excess layer of insurance or reinsurance attaches.
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The amount of loss which attaches before the next higher excess layer of insurance or reinsurance attaches.