Concurrent service — when one person holds two or more posts or duties at the same time, commonly used in companies or government offices.
部長は課長も兼務しているため多忙だ。
The director is also serving as section chief, so he is very busy.
彼女は企画部と営業部を兼務している。
She concurrently works in both the planning and sales departments.
An additional post — the extra position held (often used with する: 兼務する).
営業部長の兼務は本年度だけの予定だ。
The concurrent post of sales manager is planned only for this fiscal year.
非常勤講師の兼務先が二つある。
The part-time lecturer has two concurrent posts.