A Hall Pass is permission from your partner to have a one-time fling with someone else, often a named celebrity, without it counting as cheating.
A hall pass is an agreement between partners that one of them may have a free pass to hook up with a specific person, often a celebrity they will likely never meet.
The idea is usually playful. Naming your hall pass, the famous person you would get a guilt free night with, is a common couples game.
In rarer cases couples extend a real hall pass for a one time encounter, treating it as a bounded exception rather than open non-monogamy.
The concept only works on clear, mutual agreement. A hall pass given freely is very different from one assumed or taken without consent.
For most couples it stays hypothetical and lighthearted. When taken seriously, it calls for the same honest conversation any open arrangement requires.