Used as a prefix or noun to mean 'the whole of something' or 'in its entirety'. Common in compounds like 丸ごと (marugoto, 'whole') or 丸一年 (maruichinen, 'a full year').
丸ごとのリンゴをかじる。
I bite into a whole apple.
丸一日かけて掃除した。
I spent a full day cleaning.
noun
Slang term for money, similar to 'dough' or 'cash' in English. Often used in casual conversation.
今月は丸がないから、外食は無理だ。
I don't have any dough this month, so eating out is out of the question.
noun
castle enclosure
Only when written 丸
Historical term for an enclosed area within a castle's walls, especially the main bailey. Often seen in castle names like 本丸 (honmaru).
姫路城の本丸は広い。
The main enclosure of Himeji Castle is spacious.
noun
Kansai dialect word for soft-shelled turtle (suppon). Not used in standard Japanese.
See also: スッポン
関西ではスッポンのことを「まる」と言うことがある。
In Kansai, they sometimes call a soft-shelled turtle 'maru'.
suffix
suffix for names (ships, people, swords, etc.)
Only when written 丸
Traditional suffix attached to names of ships, infants, swords, armor, musical instruments, dogs, horses, etc. Often gives a familiar or endearing tone. Still used in modern ship names.
See also: 麻呂
日本丸は有名な帆船だ。
The Nippon Maru is a famous sailing ship.
昔の刀には「〜丸」と名付けられたものが多い。
Many old swords were given names ending in '-maru'.
Alternate kanji for 'circle', but less common than 丸 and easily confused with 円 (えん, 'yen' or 'circle').
The word まる is of native Japanese origin, with the core meaning of 'round' or 'circle'. The kanji 丸 is conventionally associated with this meaning. The suffix use likely developed from the idea of something complete or perfect, later extended to names.