Yggdrasil (now defunct since 1995) is one of the very first Linux distributions circa 1992. It was the first to distribute on cdrom and bundle dead tree documentation. It was created by Adam Richter, and is named after Norse mythology.
Yggdrasil is pronounced 'yaag - draw - sill'
Since Yggdrasil is long gone, slackware is recognized as the oldest distribution still active in the Linux community.