A session is a single collection of pseudo terminals under the management
of tmux. Each session has one or more windows linked to it. A window
occupies the entire screen and may be split into rectangular panes, each
of which is a separate pseudo terminal (the pty(4) manual page documents
the technical details of pseudo terminals). Any number of tmux instances
may connect to the same session, and any number of windows may be present
in the same session.
Признаться, я сам не знаком особо с терминологией в Screen. Даже его справочную страницу не читал.