1A circle, or anything round — the shape represented by 「○」.
正しい答えの横に丸を付けた。
I put a circle next to the correct answer.
紙に大きな丸を描いて、その中に絵を描いた。
I drew a big circle on the paper and drew a picture inside it.
公園の中央には丸い噴水がある。
There's a round fountain in the middle of the park.
2Whole, entire, or complete — used as a prefix in words like 「丸ごと」(entirely) or 「丸一日」(a full day).
リンゴを皮ごと丸ごと食べた。
I ate the apple whole, skin and all.
そのプロジェクトの完成に丸一年かかった。
It took a full year to complete that project.
3An old slang term for money; rarely heard in everyday modern speech.
あいつは丸をずいぶん持っているらしい。
Word is that he's got quite a bit of money.
4An enclosure or bailey within a castle's walls, set off by moats or stone ramparts — as in 「本丸」(the main bailey) or 「二の丸」(the second bailey).
本丸には城主が暮らす御殿があった。
The main bailey housed the palace where the castle lord lived.
5A Kansai-dialect term for the soft-shelled turtle.
この店では丸を使った鍋料理が名物だ。
This restaurant is famous for its hot pot made with soft-shelled turtle.
6A suffix traditionally attached to the names of ships, as well as childhood names, swords, musical instruments, and dogs or horses.
その貨物船は「はやぶさ丸」という名前だった。
That cargo ship was named 「はやぶさ丸」.
昔は幼い子どもの名前に「丸」を付ける習慣があった。
In the past, there was a custom of attaching 「丸」 to young children's names.