What is Verjus or verjuice?

Literally 'green juice', this is a sour juice made from sour grapes, crab apples or other unripe fruit. It was used a great deal in medieval European cookery and is undergoing a revival. It's used as a flavouring in certain condiments, and in cooking can be used in place of lemon juice. It's easier to find now and can be found in the speciality food section of most major supermarkets, or in specialist food shops.