A unit of time meaning one year (a period of twelve months). Depending on context and reading, it can also mean 'some time ago' or 'a past year'.
私は一年日本に住んだ。
I lived in Japan for one year.
一年で日本語の力がずいぶん上がった。
My Japanese ability improved a lot in one year.
その出来事は一年ほど前のことだ。
That event happened about a year ago.
ひととし（＝一年）を振り返ると、たくさんの経験があった。
Looking back on the year, there were many experiences.