Session

quote "Design and Implementation of the 4.4BSD Operating System page 109"

A session is a set of one or more process groups and may be associated with a terminal device. The main uses for sessions are to collect together a user's login shell and the jobs that it spawns, and to create an isolated environment for a daemon process and its children.

ssh-agent(1)

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In BSD one can see session and process group as well as pid and parent pid with the following command:

$ ps jax