Long form: System Administrator (concatenation of system and administrator)
Large systems of computers typically have a multiple sysadmins sharing responsibility for the servers, as the computers are often required to be online 24/7. Smaller organizations may only have one sysadmin, who is often available at all times. Other places where sysadmins may be encountered are universities, network gaming rooms, and the like.
The term SysAdmin was first used on Multics, http://www.multicians.org/ a mainframe operating system for GE (later Honeywell) mainframes. It comes from the "project" name used by system administrators.
The system administration software for Multics was written in about 1969, by Joseph F. Ossanna and Michael J. Spier. (The first Multics SysAdmin other than test users was TomVanVleck, then a member of the MIT research staff.) No one remembers now who coined the term or adopted the convention of CamelCase.
The piece of the Multics system administration code that was responsible for logging in users, roughly equivalent to Unix getty, is online at http://www.multicians.org/dialup_.html