What is "session"

One session refers to all requests a single client makes to a server.

Of Wikipedia:

A session is a semi-permanent exchange of information interactively, also known as a dialogue, a conversation or a meeting, between two or more communication devices, or between a computer and the user (see login session). The session is created or established at a certain point in time, and destroyed at a later point in time. An established communication session may involve more than one message in each direction. A session is typically but not always, monitor, which means that at least one of the communicating parties needs to store information on the history of the session in order to be able to communicate, as opposed to communication stateless, where the communication is composed of independent requests, with replies.

An established session is the basic requirement to perform a connection-oriented communication. The session is also the basic step to broadcast in mode connectionless communication. However, any unidirectional transmission does not define a session.