Aquafaba is the viscous cooking liquid left after legumes such as chickpeas have been cooked—most often the brine drained from a can of chickpeas. The word comes from Latin aqua (water) and faba (bean). Because that liquid contains dissolved starches, proteins, and other soluble plant solids, it can foam, emulsify, bind, and thicken much like egg whites.
Cooks commonly use about 2 tablespoons (30 ml) of aquafaba in place of one medium egg white, or about 3 tablespoons (45 ml) for one whole egg, in vegan meringues, marshmallows, mayonnaise, baked goods, and foamy cocktails. Chickpea and white-bean aquafaba behave most like eggs; other legumes can work with some adjustment.
Source: Wikipedia, Aquafaba