See also: Choux Paste Recipe
An éclair is a delicate, individual pastry made with chou paste (choux paste, pâte à choux, cream puff pastry dough). The dough is piped from a pastry bag in an oblong or log shape on baking pans, and baked until it is crisp and hollow inside. It is either filled from a hole made in one end, or split lengthwise and filled. The filling is traditionally a vanilla pastry cream (crème pâtissière), or whipped cream, and usually topped with a chocolate fondant or confectioners’ glaze. Other fillings include coffee and rum flavored custard, fruit flavored fillings or chestnut purée, and the topping is usually flavored the same as the filling.
As an English language word, its first appearance was in the 1706 edition of Edward Phillips's New World of English Words: 'Petits Choux, a sort of Paste for garnishing, made of fat Cheese, Flour, Eggs, Salt, etc., bak'd in a Pye_pan, and Ic'd over with fine Sugar.' It did not really come into general use until the late 19th or early 20th century.
I believe the most accurate definition is to be found in Chambers English Dictionary (1988); it defines an éclair as “a cake, long in shape but short in duration.”