Two regions from the same provider located in the same broad geography but physically separated (typically 300+ km apart). Provider updates roll out to one region first, then the other.
An AZ-900-specific (Azure) concept. Examples: East US ↔ West US, North Europe ↔ West Europe. Used for geographic disaster recovery. Azure pairs regions to ensure updates never hit both at the same time.
Region pair is a concept that can show up on the exam. Check the full guide to see it in context.