OS X Mountain Lion (version 10.8) is the ninth release of macOS, shipped on July 25, 2012 — the release that dropped "Mac" from the OS name and made the iOS import systematic: Notification Center, Messages (iMessage on the desktop), Reminders, Notes, Game Center, and AirPlay Mirroring all crossed over, with iCloud stitched through the system as the connective tissue.

Gatekeeper arrived as the security counterweight — default policies limiting apps to the App Store and identified developers — the moment Mac software distribution acquired a checkpoint. The $19.99 price and the announcement's rollout (a website and press briefings, no event) signaled the annual, low-key cadence the Mac has followed since.

Notification Center; Messages; iCloud document syncing; Gatekeeper; Dictation; Power Nap; AirPlay Mirroring; Game Center; Reminders and Notes; Twitter (and later Facebook) system sharing; and Safari 6's unified smart search field.

Mountain Lion normalized the annual Mac release and completed the iOS feature bridge Lion opened. Gatekeeper's identified-developer model became the Mac's security architecture — and its slowly tightening ratchet remains the platform's defining openness debate.