daikon radish
A large, mild white radish from East Asia, crisp and juicy raw, soft and broth-absorbing when cooked.
Daikon is sold in Asian groceries, often in Turkish or Middle Eastern shops, and increasingly in well-stocked supermarkets. A white radish from a farmers' market comes closest, though it is usually smaller and sharper.
Substitute:
White radish is a good substitute, slightly sharper when raw; for simmered dishes and soups, turnip also works, though it is sweeter and less juicy.