Yamaimo is a variety of yam also known as "mountain yam" and "mountain potato." The plant is native to Japan and the name roughly translates to
"long potato." Several people have notified me that the actual translation is "mountain (yama) potato (imo)." This root vegetable was cultivated for centuries in Japan. It was considered a prized food eaten only by nobility. Today, yamaimo is also grown in other areas of the world including many Asian countries and California.
Yamaimo roots have an irregular, bumpy shape like yams. They are about two to three feet long, however some wild roots have been measured at over forty feet. Yamaimo has a golden or tan thin skin and white, creamy flesh. Unlike yams or sweet potatoes, the flesh is very sticky and glutinous, making it unappealing to most Western tastes. However, the root is highly prized in many Asian countries. The best place to locate some yamaimo is in an Asian market. Whole and pieces of fresh yamaimo are generally available and you can also the root in powdered form as well. Store fresh yamaimo in a cool pantry for up to several weeks.
Yamaimo has a bland, mild flavor that goes well with a variety of dishes. To prepare the root, peel off the skin and cut the root into pieces. Soak the pieces in water both to prevent discoloration and avoid any itching reactions some people have when touching or eating the root. Yamaimo can be boiled, baked, and steamed. In Japan the root is also served in salads and battered and fried as tempura. It is used to flavor soba noodles, which are then called "yamaimo soba" or "tororo soba." Finely grated pieces of yamaimo are often added to dishes like pancakes (Okonomiyaki) or dumplings as a thickening agent. The root is considered an aphrodisiac and also has been used to treat a variety of ailments.
gn0sis informs me that "tororo" is the term for grated yamaimo.
Many thanks to liveforever for the kanji and greyblue and fhayashi for the correct Japanese translation!!