Also called "semi-voiced sound mark".
A Japanese diacritical mark used with hiragana or katakana. It changes the sound of the kana as follows:
Without handakuten: ha hi fu he ho
With handakuten: pa pi pu pe po
It can only be used on the kana listed above.
The handakuten looks like a small circle placed to the upper right of the kana. In small print, it is easily confused with dakuten.
See also dakuten.