Deductible is something you deduct, especially from taxable income or tax to be paid. This is the Adjective definition to it now the noun definition is,
Deductible is like an insurance policy a specified amount of money that insured must pay before an insurance company will pay a claim.
For Example if you own any kind of insurance-car, home,health, this is something that is like deductible. Now in health insurance a deductible is the amount that you as a policyholder must pay each year toward your medical expenses before the insurance company begins to pay its share.
Hope this helps :)