what is the real definition of computer server? when i do the research, some say computer that the program and provide service to client. some say individual computer on a LAN acting as service, some say not use directly - share the program. i do not know which one? help me.

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

meaning like it just a computer to serve the client like network? is that true?

meaning like it just a computer to serve the client like network? is that true?

Yes, a kind of computer, that serves clients requests. For example websites.

Word 'Network' is very general and there could be an example of breaking the meaning. For example P2P network, but here is each computer a server and client at the same time.

Please note, that there could be someone that would give you an opposite that would break this meaning.