One part out of N; the fraction 1/N (e.g. one-fifth, one-tenth). Used after a number N to indicate one of N equal parts.
クラスの四分の一は留学生だ。
One quarter of the class are foreign students.
ケーキを三分の一だけ食べた。
I ate only one third of the cake.
この確率は百分の一にすぎない。
This probability is only one in a hundred.
「二分の一」は英語で「one half」と言う。
"二分の一" is said as "one half" in English.