What is purchasing power?
The purchasing power of money is the amount of goods or services that can be purchased with a certain amount of money at a certain time. For example, if you have $20 and a gallon of gas costs $4, the purchasing power of that $20 is five gallons of gasoline.
Having said that, purchasing power is more often used as a general financial concept, as opposed to how much of a specific item you can buy. For example, you might hear about the purchasing power of the average American's annual salary, or how far $1 million in retirement savings will go.