Definitions

Annual interest rate (APR): The yearly cost of a mortgage, including interest, mortgage insurance, and the origination fee (points), expressed as a percentage. Maximum APR is 20%.
Mortgage length (years): Total length, or term, of your original mortgage in years. Most common lengths are 30 years and 15 years.
Original mortgage amount: The original amount financed with your mortgage. Not to be confused with the remaining balance or principal balance.
Additional monthly payment: Your proposed extra payment per month. This payment will be used to reduce your principal balance.
Scheduled payment: Monthly principal and interest payment, or PI, based on your original mortgage amount, term and interest rate.
Accelerated payment: Scheduled payment plus additional monthly payment.
Total savings: Total amount you would save in interest if you made the accelerated payment until your mortgage was paid in full.