Individual responsible for the installation and maintenance of an information system, providing effective information system utilization, adequate security parameters, and sound implementation of established Information Assurance policy and procedures.
Sources:
NIST SP 1800-16B under System Administrator from CNSSI 4009-2015
NIST SP 1800-16C under System Administrator from CNSSI 4009-2015
NIST SP 1800-16D under System Administrator from CNSSI 4009-2015

  A person who manages a computer system, including its operating system and applications. A system administrator’s responsibilities are similar to that of a network administrator.
Sources:
NIST SP 800-44 Version 2 under System Administrator

  A person who manages a computer system, including its operating system and applications. Responsibilities are similar to that of a network administrator.
Sources:
NIST SP 800-45 Version 2 under System Administrator

  Individual or group responsible for overseeing the day-to-day operability of a computer system or network. This position normally carries special privileges including access to the protection state and software of a system.
Sources:
NISTIR 4734 under System Administrator
NISTIR 7316 under System Administrator