one-or-the-other rule
nounUK /wʌn ɔː ði ˈʌðə ruːl/
US /wʌn ɔːr ði ˈʌðər ruːl/
A rule that says you must pick just one of two things, not both.
The one-or-the-other rule means you can have cake or ice cream, but not both.
OTHER MEANINGS
2. A logic principle where only one of two statements can be true.
Under the one-or-the-other rule, the light is either on or off, never both.
3. A game rule forcing players to choose a single action each turn.
The board game uses a one-or-the-other rule: move or draw, not both.
4. A fairness guideline letting someone take only one item from a pair.
At the party, the one-or-the-other rule let each kid take one cookie or one candy.