Confidentiality
The right eyes only. Encryption, access control, and not leaving copies in the wrong tray.
Confidentiality is the promise that data is shown only to entities that are allowed to see it. Identity, classification, and encryption are the usual machinery. Gossip in Slack is not.
It fails when a mailbox is shared, a backup is open, a screen is in a cafe, or a contractor keeps last year's export.
Pair it with integrity and availability. Perfect secrecy on a system nobody can use is not a program.
Fact source: ASD's ACSC glossary.