む, in hiragana, or ム in katakana, is one of the Japanese kana, which each represent one mora. Both hiragana and katakana are made with three strokes, they represent [mu͍]. The kanji 無, read "mu", represents "none", nothingness, or the state of oblivion upon death, if written alone.
In the Ainu language, ム can be written as small ㇺ, which represents a final m sound. This, along with other extended katakana, was developed by Japanese linguists to represent Ainu sounds that do not exist in standard Japanese katakana.