An amount for which the insured is their own insurer; in the event of a claim the insured bears the corresponding part of the claim with their own assets. This serves to reduce the amount of the premium of an insurance policy.
For example, an automobile insurance policy establishes an excess of 300 euros. If the vehicle suffers an accident with damages valued at 250 euros, the insurance company will not indemnify the insured. Up to a value of 300 euros (the excess) any damage must be paid for by the insured party.
If the loss is valued at 1,000 euros, the insurer will indemnify 700 euros, which is the value above the excess. The insured will still have to pay the excess themselves.