Why go
It is worth recognizing for the contrast of airy, steam-puffed choux pastry with a soft filling and a smooth topping. The name is sometimes applied loosely elsewhere, but a true éclair is defined by choux dough, which sets it apart from Long John-style doughnuts.
The full story
The éclair belongs to the choux-pastry family, where steam puffs the dough during baking to create a hollow interior. That space is filled with cream or custard, then topped with a glaze. The structure matters because pastries that look similar can be made differently. In North America, some chains market Long John doughnuts as éclairs or éclair doughnuts, but Long Johns use yeast-risen or batter-derived dough rather than choux. They are commonly filled with vanilla pudding or custard and topped with cake icing. The distinction is useful when comparing pastry traditions: an éclair’s light shell comes from steam-puffed choux, while a doughnut begins with a different kind of dough altogether. On a French pastry counter, the elongated choux shell, filling and glaze are the practical clues to look for.