A SysOp is short for "system operator", and is a commonly used term for the administrator of a bulletin board system (BBS) or special-interest area of an online service or, historically, the operators of any computer system, especially a mainframe computer.

Usage peaked in the early 1990s, as the term was actually mostly used strictly to refer to BBS operators; someone doing equivalent functions on the Internet is primarily referred to as an "admin", short for "administrator".

A Co-SysOp was a user who was granted certain admin privileges on a BBS. Generally, they would help validate users and monitor forums.

On the Wikimedia Foundation projects like the Wikipedia Encyclopedias, administrators are called sysops, while they do the duties of a co-sysops. This is discussed more on particular Wikipedia policy pages.