Verjuice or verjus derives from the phrase 'green juice' and was widely used in the Middle Ages. Having fallen from favour, to the point where few people have even heard of it, it is now experiencing a fashionable revival.
It is usually a sour, acidic juice extracted from unripened grapes and therefore is often produced by wine makers, especially in France, Australia and Spain. In Britain verjuice used to be made with crab apples.
Verjuice is available in some delicatessens and online from suppliers and in larger supermarkets too. Use lemon juice or sherry as a substitute - it won't be quite the same but will bring your recipe some of the necessary souring quality verjuice provides.
Use verjuice any place you would use white wine or dry sherry in cooking, for example sauces, stews and risottos, and especially for deglazing.
A traditional ingredient of made mustards, it is good for fish stock or court bouillon and makes delicious fruit compotes.