PMT is a function in Microsoft Excel that calculates the payment for a loan based on constant payments and a constant interest rate. The function takes several arguments, including the interest rate, the total number of payments, and the present value or total amount that a series of future payments is worth now. The payment returned by the PMT function includes principal and interest but no taxes, reserve payments, or fees sometimes associated with loans. The PMT function can be used to calculate the monthly payment for a loan, and it is commonly used in financial modeling and analysis.