General:
* system: A set of interacting or interdependent components forming an integrated whole.
* systematic: Done or acting according to a fixed plan or system; methodical.
* systematize: To arrange or organize according to a system.
* systematically: In a systematic manner.
* syndrome: A group of symptoms that occur together and characterize a particular disease or condition.
Computer Science:
* sysadmin: A person responsible for the maintenance and operation of a computer system.
* syscall: A request from a computer program to the operating system for a service.
* sysinfo: Information about the system, such as hardware and software.
* syslog: A system for logging events and messages in a computer system.
* sysop: The operator of a computer system, especially a bulletin board system (BBS).
Other:
* systema: A type of dance or music, especially in the Balkan region.
* systema naturae: A system of classification of living things, developed by Carl Linnaeus.
* syssarcosis: A type of articulation in which bones are connected by fibrous tissue.
* sysygy: A straight-line configuration of three celestial bodies, such as the Sun, Earth, and Moon.
Note: Some words may have variations in spelling, such as "systema" or "systema naturæ."
This is not an exhaustive list, as the "sys-" prefix can be used in a variety of contexts.