Insurance: the amount you have to pay before the insurance company pays.
In effect it is an amount subtracted from any payout.
Example: let's say your car is damaged and it costs $2,000 to repair. Your insurance policy has a $200 deductible, so they may only pay $1,800 (you have to pay the first $200).
Note: this is very similar to an Excess, but an Excess will not reduce the maximum payout.
Example: let's say the maximum payout is $5,000, and there is $2,000 damage, a deductible of $200 would only pay out $1,800, and an excess of $200 would ALSO pay out $1,800.
BUT if the car was only insured for $1,000, a deductible of $200 would only pay out $800, but an excess would pay the full $1,000.