Ma (마) (麻) is a root vegetable (Dioscorea opposita) used in Korea as a traditional medicine and for food. Alternate names are Dioscorea batatas, D. japonica, and D. oppositifolia. It is also used in Japan, where it is called naga-imo or yama-imo, as well as in China.
Dioscorea opposita is an exception to the rule that yams must be cooked before consumption (due to harmful substances in the raw state). In Japanese cuisine, it is eaten raw and grated, after only a relatively minimal preparation: the whole tubers are briefly soaked in a vinegar-water solution, to neutralize irritant oxalate crystals found in their skin. The raw vegetable is starchy and bland, mucilaginous when grated, and may be eaten plain as a side dish, or added to noodles.
In Japan the jelly-like substance made from grating the yam, tororojiru (とろろ汁), is often served in, or alongside, a number of other dishes. Interestingly, perhaps, this was widely used in the Edo period as a personal lubricant for homosexual activities, and it was thus considered improper for it to be eaten by a woman. This aversion also derives from the loud slurping sound one makes when eating it, which is considered to be un-ladylike.