Japanese mustard. からし、芥子。
Made from the crushed seeds of leaf mustard (Karashina, Brassica juncea). Came in the 10th c. from China to Japan, after which new types were developed.
Karashi is usually sold in powder form or paste form in tubes.
Karashi is used as a condiment with tonkatsu, oden, and shumai. These are all "new" dishes which entered Japanese cuisine in the 20th century, which again serves to show that in the case of traditional dishes no spices were used.
Karashi is also mixed with other condiments, for example with mayonnaise to make "karashi mayonnaise."
It is also used to make pickled Japanese eggplant, called karashi-nasu - a great side-dish with sake.
One of Kumamoto's regional dishes is karashi renkon - slices of lotus root of which the openings have been filled with karashi.