A small cup or dish with slots or a gap in its rim that holds back the yolk of a cracked egg while the white runs through into a bowl below. It exists because a trace of yolk in egg whites stops them whipping to full volume for a meringue or soufflé, and because a broken yolk at the point of separating by hand wastes the white.
Cooks who are confident separating eggs between the shell halves or through their fingers rarely use one, but it is quick and clean when a recipe needs a dozen whites, and it keeps shell fragments out of the bowl. Separate each egg over a small bowl first so one bad yolk does not spoil the batch.