Availability
Authorised users can actually use the system when the work needs it. Uptime theatre without backups is a bet.
Availability is the assurance that systems stay reachable and usable for the people and processes allowed to use them. Capacity, redundancy, patching that does not brick the fleet, and restores you have practised all sit here.
Denial-of-service is the loud failure. The quiet ones are expired certificates, a single cloud region, and backups that restore into a smoking crater.
Do not trade it away for confidentiality theatre. A locked vault nobody can open during an incident is an outage with better branding.
Fact source: ASD's ACSC glossary.