Sauce Bigarade is a classic French sauce which features the flavour of bitter orange. It is one of the few French sauces which can be classed as sweet and sour.
The sharpness of the bitter orange cuts through both the sweetness of the sauce, and any fatty taste of the meat (typically duck) it is served with.
The most well known use of it is in Caneton à la Bigarade (aka Duck à l'orange.) It also appears in recipes such as Truite (trout) à La Bigarade.
Classically, the sauce is based on a gastrique (carmelized sugar mixed with wine vinegar) to which is added veal stock ("blond de veau", aka "Fonds de veau") or demi-glace sauce, along with the juice and blanched rind of a bitter Seville orange.
The issue with the classic version is that either suggested base sauce to add -- "blond de veau" or demi-glace sauce -- is a major undertaking in itself that can take a day or two to prepare, and that really assumes you have underlings in a restaurant kitchen making it for you.
If you making make the sauce for Duck à l'orange, Larousse Gastronomique (1977), while acknowledging the classic base-sauce method, seems to prefer instead (listing it first before the classic method) that you instead braise the duck, and use the braising juices (simmered down) as the liquid. Gordon Ramsay suggests good quality chicken stock as the liquid.
Modern home suggestions for Sauce Bigarade often base it on a roux, skip the gastrique, and add an orange liqueur both as the sweetener and to intensify the orange flavour (Gordon Ramsay bases his on a gastrique, and rounds out his version with port instead.)
"Bigarade" is the French term for Seville Oranges.