Yam
Japanese mountain yam (nagaimo or yamaimo), a long, pale-skinned tuber that turns into a thick, slippery paste when grated raw.
Caution: Only Japanese nagaimo or yamaimo may be eaten raw, other yams must be cooked, and the skin often irritates the hands during peeling and grating, so wear gloves or rinse your hands with vinegar afterwards.
In Japanese cooking, yam means the tubers of Dioscorea species, chiefly the long, watery nagaimo and the firmer, stickier yamaimo, not the orange sweet potato often sold as yam in North America, and not the starchy African yams. The flesh is white, mild and faintly sweet, crisp like radish when cut raw into sticks, but stringy and slippery when grated. That texture decides how it is used: grated yam, called tororo, is spooned raw over rice or noodles or serves as a light binder, for instance in okonomiyaki batter. Cooked, the tuber turns soft and potato-like with a slight bite. Look for it in Asian grocery stores, usually chilled and wrapped in film, sometimes frozen and already grated.
Substitute:
For raw tororo there is no substitute, since no other common tuber becomes as slippery when grated and can also be eaten raw. In okonomiyaki batter you can leave it out and bind with an extra egg and a little potato starch, though the pancake will be denser and less fluffy.