In a generic context (without specifying a specific application or industry), what is meant when interest rates are quoted as an APR?

APR is the effective interest rate, compounded over 12 months

APR implies an annual rate, but is ambiguous about how this rate is calculated.

APR is the simple interest rate, compounded over 12 months

APR implies a periodic rate, but is ambiguous about how long this period is.

APR is the simple interest rate over 6-months, doubled