"small, oblong cake filled with cream or custard and glazed or iced," 1861, from French éclair, literally "lightning," from Old French esclair "light, daylight, flash of light," verbal noun from esclairare "to light up, illuminate, make shine" (12c.), formerly esclairer, ultimately from Latin exclarare "light up, illumine," from ex- "out" (see ex-) + clarus "clear" (see clear, adj.).