Udon (うどん) is a style of noodle popular throughout Japan. It is used in a variety of Japanese cuisines and is much thicker than Soba, another type of Japanese noodle. It is also made with wheat, and can be served with soy sauce or, more commonly, in a bowl of soup with tofu, some meats and a salty broth.
Udon is most commonly found in soups. The type and style of soup can vary greatly by region, but these soups are generally salty, sometimes slightly sour, made with a dark soy sauce and possibly some tofu, vegetables, and meats. The broth itself may have some meat base, though not necessarily. Many of the soups have some type of Japanese curry as an ingredient.
Udon may also be served in instant noodles, and there are many easy to find udon soup stands on the streets of Japan. These instant noodles are far more common in Japan than worldwide, where ramen noodles are found more often.
As the popularity of using noodles in foods spread during the Edo period, and it was not until that time that Udon began to become a regular food item found in most Japanese cuisine. It was still seen in many meals, but Soba had been the noodle of choice and the Edo period saw the first emergence of Udon as an acceptable alternative.
Udon was likely introduced by China in the 8th century. It is thought that a Buddhist priest visited China and came back with the method of creating the Udon noodle. The original Udon noodle was very thick, ranging from 2 to 3 centimeters in diameter, but over time the size of the noodle began to shrink in order for the quantity and texture of the noodles to improve.