Mustard: a strongly flavored condiment, in powder or paste form; in Japanese cuisine it's often served with sashimi, oden, pickles, etc. Usually written in kana.
からしを少しだけお好み焼きにつけて食べた。
I ate the okonomiyaki with a little mustard.
からし漬けは独特の辛さがある漬物だ。
Karashi-zuke is a pickle with a distinctive spiciness from mustard.
からしを入れすぎると辛くて食べられないことがある。
If you add too much mustard, it can be too spicy to eat.