noun
Used when counting people in order, such as the second child, second customer, or second person to do something.
彼は二人目の子供です。
He is the second child.
二人目の客が来た。
The second customer came.
Means 'third person', continuing the counting pattern.
Compound of 二人 (ふたり, 'two people') + 目 (め, ordinal suffix).