noun
relief worker; shift replacement; person taking over a shift
Refers to a person who replaces another on a shift or duty, often in work, guard, or service contexts.
夜勤の交代員が来るまで待ってください。
Please wait until the night shift relief worker arrives.
彼は警備の交代員として働いている。
He works as a relief security guard.
当直 means being on duty (especially night duty), whereas 交代員 is the person who relieves that duty.
Compound of 交代 (shift, alternation) and 員 (member, person).