Server

In an application which runs across a communication network, the server is the end which provides the service (hence the name); the client at the other end makes contact with the server to request the service.

While clients may come and go (in the sense of being started and shut down), servers (or a service housing which is prepared to start a server when a request comes in) generally are always operational, waiting for a request to come in (since the requests usually arrive asynchronously).