Something that requests data from a server is known as a

It is  called a client.

Explanation:

Any entity that request data from a centralized node in a network (which is called a server, because once received a request, it replies sending the requested piece of data back to the requester) is called a client.

The server can adopt different names based on its role: It can be a file server, an application server, a web server, a mail server etc.

This sharing information paradigm, where the resources are located in one host (server) to be distributed to many hosts (clients)  is called client-server model.

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