Karashi (辛子, からし) is a Japanese hot mustard made of a mixture of crushed mustard seeds of Brassica juncea and horseradish. It’s used as a condiment or as a seasoning in Japanese cuisine, such as oden, gyoza, and tonkatsu. Karashi is usually sold in powder form or paste form in tubes (see the photo).
Karashi is a spicier and more potent condiment than mild yellow mustard.
Substitute: The closest substitute would be English mustard and Chinese hot mustard powder.