noun
a full year; the whole year
Emphasizes the completeness of a one-year period, often used in contexts like 'for a whole year' or 'after a full year'.
彼は丸一年、海外で働いていた。
He worked abroad for a full year.
丸一年経って、やっと結果が出た。
After a whole year, the results finally came out.
一年 simply means 'one year' without the emphasis on completeness that 丸一年 carries.
Compound of 丸 (まる, 'whole, complete') and 一年 (いちねん, 'one year').