The setting of an interest rate for a predetermined future period.
For example, the periodic re-setting of the interest rate on a floating rate loan.
The use of derivative instruments such as Forward rate agreements (FRAs) for hedging purposes, to effectively fix a hedged rate.
A fixing instrument (or fixing derivative) is one which hedges an exposure by effectively fixing a hedged rate for it.
Contrasted with an insurance-type instrument, such as an option.