Apple's OS X El Capitan is version 10.11 of the Mac operating system released in September 2015. El Capitan, named for the iconic landmark in Yosemite National Park, builds on the groundbreaking features and beautiful design introduced in OS X Yosemite, refining the experience and improving performance in lots of little ways that make a big difference.
Download OS X (El Capitan) from Apple. (*Within Mac App Store)
-- OS X (El Capitan): Upgrade Checklist
OS X (El Capitan): What's New?
OS X (El Capitan): Support
OS X (El Capitan): Communities
Referenced from: Apple.com