A boy; a male (often used to label gender/age categories such as male students or male athletes).
小学校の男子は校庭で元気に遊んでいる。
Boys at the elementary school are playing energetically on the playground.
入試では男子と女子で別々に試験が行われた。
In the entrance exam, boys and girls were tested separately.
彼は中学時代、優秀な男子生徒だった。
He was an outstanding male student in junior high.
A man or male (used in formal contexts or category labels such as men's restroom, men's team).
オリンピックの男子100メートルで日本チームが出場する。
The Japanese team will compete in the men's 100 meters at the Olympics.
この建物には男子更衣室と女子更衣室がある。
This building has a men's changing room and a women's changing room.