TVM = Time value of money, the basic concepts of what these financial calculators are all about.  PV = Present Value (beginning investment values, or how much something is worth today).  PMT = Payments (constant contributions or withdrawals each compounding period, usually added to or subtracted from the scenario monthly).  I = Compound interest rate (interest rate or growth rate that is applied at each N compounding period).  N = Number of compounding periods (how many times the calculations are done that compound money at the given interest rate, usually in months or years. E.g., If you have $100 and you compound, or add, 10% annual interest, then you'd have $110 as a future value over this one compounding period. If N=2, then it would compound twice, and your ending value would be $121, which is $110 plus 10%).  FV = Future value (usually the result, or the amount of money accumulated over N compounding periods).  P = Principal, or how much money is being lent or borrowed.  ARM = Adjustable Rate Mortgage.  APR = Annual Percentage Rate.