A person in charge; a senior staff member responsible for a section, task, or project.
彼は営業部の主任に昇進した。
He was promoted to section chief of the sales department.
会議では主任が報告を行った。
At the meeting, the person in charge gave the report.
プロジェクトの主任として全体の進行を管理する。
As the project manager/chief, (he/she) manages the overall progress.