noun
monk; friar
Specifically refers to a Christian monk or friar, not a Buddhist monk (which is usually 僧 or 坊さん).
その修道院には十人の修道士が住んでいる。
Ten monks live in that monastery.
修道女 is the female equivalent, meaning 'nun' or 'sister'.
From 修道 (shūdō, 'religious training' or 'monasticism') + 士 (shi, 'person of a certain profession'). The term was coined to refer to Christian monastics.