Client vs. Server terminology

In TCP, it's determined by the side that initiated the connection. The client initiates a connection, and the server listens for and accepts a connection. Once connected, data can flow both ways.

Definitions:

Server-side refers to operations that are performed by the server in a client–server relationship in computer networking. Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user's local computer or workstation.

Client-side refers to operations that are performed by the client in a client–server relationship in a computer network. Typically, a client is a computer application, such as a web browser, that runs on a user's local computer or workstation and connects to a server as necessary.