A Client is a software application that contacts and obtains data from a Server software application. A Client makes a request and the Server fulfills the request. An example of a Client would be an e-mail program connecting to a mail server or an Internet browser client connecting to a web server.

In the case of OPC, an HMI or Data Historian typically would typically use OPC to acquire automation data from an OPC Server.

See also: Client/Server, Server