A batsu, drawn as an "X", is the Japanese symbol for "wrong" or "incorrect".

This is in opposition to maru, drawn as "O", which is used to connote roundness and correctness. A sankaku, or triangle-mark, is used as a sign of partial completion.

Sometimes, people play a batsu game--in which if someone fails a dare or loses at jan ken pon, that loser must suffer some penalty or do some embarassing act/favor.