Cold udon noodles. 冷やしうどん。
Udon are thick, white wheat noodles. They are made from wheat that is mixed with salt and water, kneaded, stretched and sliced. Udon noodles came to Japan from China somewhere in the 8th c. They were a staple food long before soba arrived.
[Hiyashi udon. Photo Ad Blankestijn]
In contrast to soba, udon is more popular hot than cold. But in summer, cold udon is not uncommon. Hiyashi udon is eaten with a dipping sauce and often covered with sliced negi and flakes of tempura batter.
A variant using thinner wheat noodles is Hiyamugi.