What is the definition of a deductible in terms of health insurance?

There are deductions in the form of exclusions in health insurance.The exclusions are for lst year, 2nd year, showing the details of diseases in the policy bond itself. Even for Diabetes as "Pre-existing", few companies gives coverage only after 48 months. Deductions are there when the Insurance Company or T.P.A. considers few expenses viz. Aaya charges, cost of bandage, ambulance fare etc. shown in the claimed amount, to be not within the purview of health insurance coverage.