What is Annual Percentage Yield?

Legal Definition
Annual percentage yield (APY) is a normalized representation of an interest rate, based on a compounding period of one year. APY figures allow for a reasonable, single-point comparison of different offerings with varying compounding schedules. However, it does not account for the possibility of account fees affecting the net gain. APY generally refers to the rate paid to a depositor by a financial institution, while the analogous annual percentage rate (APR) refers to the rate paid to a financial institution by a borrower.

To promote financial products that do not involve debt, banks and other firms will often quote the APY (as opposed to the APR because the APY represents the customer receiving a higher return at the end of the term). For example, a CD that has a 4.65 percent APR, compounded monthly, for 8-months would instead be quoted as a 4.75 percent APY.
-- Wikipedia
Legal Definition
The way compounded interest is quoted on an investment. It assumes a 365 day period.