Nothing. All of those terms refer to the same payment. The IRS changed the way they referenced the payments throughout 2020. First the money was called a stimulus payment but later was given the more formal name of Economic Impact Payment. The Recovery Rebate Credit is how the payment is referred to on the federal tax return. The stimulus payment or Economic Impact Payment – whichever name you prefer - is an advance of the Recovery Rebate Credit, but all names refer to the same amount of money.