Fried wheat noodles. The thick udon-noodles are stir-fried with pieces of meat (in the photo below instead pieces of chikuwa are used) and slivers of vegetables as cabbage, onion, carrot and beansprouts. Also the Okinawan bitter lemon called goya can be used. The noodles are flavored with soy sauce and Japanese Worcestersauce. Beni-shoga (pickled ginger) is often served on the side.
This dish was invented in 1945 by the restaurant Darumado in Kokura, on the southern island of Kyushu. The restaurant normally made yakisoba, but as Chinese soba-noodles were unavailable due to the war, the owner tried to make a similar dish with dried udon noodles instead.
Yaki-udon is still a popular dish in Kokura, as this site promoting it shows. But more than that, yaki-udon has now become a fixed part of popular Japanese cuisine. You find it often in izakaya and home cooking as well.
[Photo from Flickr by Ad Blankestijn (Ganjin)]
In Japanese: 焼きうどん