Mutually exclusive
Options where choosing one automatically means the others can't be chosen.
Definition
Options are mutually exclusive when only one can be selected at a time. Radio buttons and segmented controls work this way. Testers check that selecting one option always deselects the previous one and that it is never possible to have zero or two selected (unless designed).
In practice
In a shipping options list, choosing "Express" must deselect "Standard". If both look selected, that is a bug.
Learn it in context
Lessons that use this term.