A fully-amortizing loan with a “fixed” interest rate over the life of the loan.
A fixed-rate mortgage is a fully-amortizing loan with a “fixed” (unchanging) interest rate over the life of the loan. In other words, the note rate is set and will remain constant (neither increase nor decrease) throughout the life of the loan.
The interest rate can still be adjusted if the homeowner decides to refinance into another rate down the line, but the fixed rate itself will not independently adjust during its term. The other rate option at the opposite end of the spectrum is an adjustable-rate mortgage (ARM).