Interest rate swaps.
The fixed leg of an interest rate swap is a predetermined series of notional fixed interest payments, exchanged for a series of floating interest payments, determined over time by the reference rate.
In practice the interest rate swap is settled for difference, so these payments are notional.
The fixed leg is also sometimes known as the fixed rate leg.
When interest rate swap prices are quoted, the two-way prices quoted are for the fixed leg rate payable or receivable by the market taker.
(The market taker takes the worse side of the two-way price.)