Verjus is a juice made from unripe green grapes and is used for deglazing pans, sauces, and in salad dressings.
Verjus has been a staple of French cuisine since the Middle Ages. The name itself comes from the Medieval French vertjus, meaning "green juice." In Syrian cuisine, it's called "husroum," and in Persian cuisine, it's "abgora."
Verjus can be used in place of wine or vinegar in many recipes. It's often found in specialty food stores, and can easily be found online.