Yosemite is bloated trash. I'm always sorry when I encounter it on someone's Mac.
OS X Yosemite (version 10.10) is the eleventh major release of OS X, Apple Inc.'s desktop and server operating system for Macintosh computers.
OS X Yosemite was announced and released to developers on June 2, 2014, at the WWDC 2014 and it was released to public beta testers on July 24, 2014. Yosemite was released to consumers on October 16, 2014. Following the California landmark-based naming scheme introduced with OS X Mavericks, Yosemite is named after the national park. More: en.wikipedia.org.