1. A computer, or a software package, that provides a specific kind of service to client software running on other computers. The term can refer to a particular piece of software, such as a WWW server, or to the machine on which the software is running, e.g. "Our mail server is down today, that's why e-mail isn't getting out." A single server machine can (and often does) have several different server software packages running on it, thus providing many different servers to clients on the network. Sometimes server software is designed so that additional capabilities can be added to the main program by adding small programs known as servlets. From Matisse

2. A program which provides some service to other (client) programs. The connection between client and server is normally by means of message passing, often over a network, and uses some protocol to encode the client's requests and the server's responses. From Linux Guide @FirstLinux

3. n. A kind of daemon that performs a service for the requester and which often runs on a computer other than the one on which the requestor/client runs. A particularly common term on the Internet, which is rife with `web servers', `name servers', `domain servers', `news servers', `finger servers', and the like. From Jargon Dictionary