One yen (the Japanese currency unit, ¥1).
財布の中に一円玉が一枚だけ入っていた。
There was only one one-yen coin in my wallet.
お釣りで一円玉を受け取った。
I received a one-yen coin as change.
価格は端数を切り捨てて一円単位にした。
The price was rounded down to the nearest one yen.
Used adverbially/suffixally as 'throughout' or 'all over' an area (e.g., 町一円 = throughout the town).
台風で市内一円の停電が起きた。
The typhoon caused power outages throughout the city.
山の一円が霧に包まれていた。
The entire mountain area was shrouded in fog.
その噂は村一円に広がった。
The rumor spread throughout the village.