Lets say, that the [use(r) or users] or computers use/access the services of the computer, but will never physically touch the computer. The "server" is from "to serve" which can be translated to "to serve others".
The reply/definition/answer depends on who you ask (what branch) and the knowledge of the person.
The definition could be like:
Server is a machine, that serves/provide services to others.(1) A server can also use services of others.(2)
(1) - emails, WWW; (2) - backups