›MANAGEMENTone part of a large organization, such as a company or university, that deals with a particular area of work, business, study, etc.: run/manage a departmentthe sales/human resources/finance departmentHe was previously a lecturer in the pharmacy department of Queen's University.a department director/supervisor
›COMMERCEone part of a large store that sells many different types of goods: the book/electrical/menswear departmentShe has a Saturday job in the footwear department of a large city centre store.
›GOVERNMENTone part of a government that is responsible for a particular part of its work: the Department of Education/Health/Justice
be sb's departmentinformal›to be something that a particular person knows a lot about or is responsible for: You'll need to ask Joe about that - it's more his departmant than mine.