OS X Mavericks (version 10.9) is the tenth release of macOS, shipped on October 22, 2013 — free, for the first time, ending the paid-upgrade era in one keynote line ("Today we announce a new era of the PC"). The naming changed eras too: the cats retired for California places, starting with the surf break.

The release itself was an engineer's year: timer coalescing, App Nap, and compressed memory stretched battery life measurably; Finder gained tabs and tags; and iBooks and Maps crossed from iOS.

Free distribution; Finder tabs and tags; multi-display improvements (menu bars and Docks everywhere); timer coalescing, App Nap, compressed memory; iBooks and Maps; iCloud Keychain; and interactive notifications.

Free OS upgrades reset the industry's economics — Windows followed within two years — and pushed the Mac fleet onto current releases, simplifying security and developer targeting permanently. The efficiency work quietly defined the modern MacBook's all-day battery.