1. A server is a computer program that provides services to other computer programs (and their users) in the same or other computers. The computer that a server program runs in is also frequently referred to as a server. That machine may be a dedicated server or used for other purposes as well.
2. In a technical sense, a server is an instance of a computer program that accepts and responds to requests made by another program, known as a client. Less formally, any device that runs server software could be considered a server as well. Servers are used to manage network resources.
http://whatis.techtarget.com/definition/server (1.); https://www.computerhope.com/jargon/s/server.htm (2.)
Author: Tim Röder