An éclair is an oblong pastry made with choux dough filled with a cream and topped with icing. The dough, which is the same as that used for profiterole, is typically piped into an oblong shape with a pastry bag and baked until it is crisp and hollow inside. Once cool, the pastry is then filled with a vanilla-, coffee- or chocolate-flavoured custard (crème pâtissière), or with whipped cream, or chiboust cream; and then iced with fondant icing. Other fillings include pistachio- and rum-flavoured custard, fruit-flavoured fillings, or chestnut purée. The icing is sometimes caramel, in which case the dessert may be called a bâton de Jacob. (source)