You'd go to an encyclopedia to find out the history, origins, detailed variations, regional specialities and so on about the topic of éclairs not to understand what an éclair itself is. A dictionary should perfectly well describe a thing without needing to pull out an encyclopedia and if you look at a updated/modern version of the Chambers Dictionary you'll get just such a definition: "A long cake of choux pastry with a cream filling and chocolate or coffee icing."