42 C.F.R. § 406.3
First month of eligibility means the first month in which an individual meets all the requirements for entitlement to hospital insurance except application or enrollment if that is required.
First month of entitlement means the first month for which the individual meets all the requirements for entitlement to Part A benefits.
Insured individual means an individual who has the number of quarters of coverage required for monthly social security benefits.
Quarter of coverage means a calendar quarter that is counted toward the number of covered quarters required to make the individual eligible for monthly social security benefits. A quarter is counted if during that quarter (or that calendar year) the individual earned a required minimum amount of money. (For details, see 20 CFR part 404, subpart B.)
42 C.F.R. §406.3