session
also: login session, SID, session ID
A session is a login instance that groups related processes and terminal connections together, typically starting when a user logs in and ending when they log out.
A session represents a complete user login environment. When you log in via SSH, a graphical desktop, or a terminal, a session is created that tracks your shell process, any child processes you spawn, and your terminal settings. The session persists until you explicitly log out or the connection is terminated.
Sessions are managed by the operating system and are identified by a Session ID (SID). All processes within a session share the same controlling terminal and receive the same signals (like SIGHUP when disconnected). For example, when you SSH into a server and run bash, that bash process starts a new session.
In graphical environments, a session encompasses your entire desktop experience—window manager, applications, and background services. You can also have multiple independent sessions on the same system, such as multiple SSH logins from different machines or multiple virtual terminals (TTYs).