Plain meaning
Start with the shortest useful explanation before going deeper.
The real rate of return earned on a deposit or investment over one year, including the effect of compounding interest. Unlike APR, APY accounts for interest being reinvested and earning additional interest over time. In DeFi, APY figures are often projected from short-term returns and can fluctuate significantly. A 10% APY means a deposit grows by 10% over a year if rates remain constant and rewards are continuously compounded.