What is the meaning of ○?

A circle.

A female family member in a family tree.

An unidentifiable or indeterminate segment.

Oxygen (Daltonian symbol).

A white stone.

A split.

circle

correct, check mark, good

Placeholder for a kanji or kana.

1. Used to indicate a space to be filled in.

   Used to indicate a space to be filled in.

   Used to censor or obscure individual characters.

   大安, the fifth day of the 六曜 (rokuyō) calendar. Good luck.

   Source: wiktionary.org