Zwieback is a hard, dry, slightly sweet bread that has been baked, sliced, and then baked a second time until it is crisp all the way through. Because it is so dry it keeps for a long time and does not go mouldy, and because it is hard it dissolves slowly in the mouth. That last quality is why it is famous as a baby food: parents in America and Europe give a piece to a baby who is cutting teeth, so the baby can chew on something firm without it breaking into dangerous pieces. It is also given to people recovering from a stomach illness, for the same reason as plain toast — it is bland and easy on the stomach. The word is uncountable in normal use: a piece of zwieback, some zwieback, a box of zwieback. It is not an everyday word in most English-speaking countries; you meet it on packaging, in baby-care books, and in recipes, more than in conversation. In Britain people would more likely say rusk, which is nearly the same thing. In Taiwan the closest familiar thing is 烤饅頭片 or the baby biscuits sold as 磨牙餅乾, though those are usually softer.

（二次烘烤的）硬脆麵包片、烤麵包乾；給嬰兒的那種常說「磨牙餅乾」或「寶寶餅乾」，英式英文則多用 rusk