millet
Small, round, yellow grains of a cereal, sold hulled; mild and slightly nutty when cooked, fluffy or porridge-like depending on the amount of water.
Hulled millet is found in most supermarkets and health food shops in German-speaking countries, while in English-speaking countries it is more likely to be in health food stores, the wholefoods aisle or online. If you can't get it, fine cornmeal or quinoa come closest, though corn tastes sweeter and quinoa stays more distinct.
Substitute:
For porridge, fine cornmeal works, turning out a little sweeter and smoother; as a side dish, quinoa will do, though it stays more distinct and has a faintly grassy note.