Yggdrasil
The Norse tree which held up the world and the heavens; its roots led to the underworld. It provided the complex network of the universe.
Yule
The pagan winter sabbat. Also called Winter Solstice. [For more information see http://www.spiritonline.com/wicca/3.html]
Yule Log
The traditional log burned at Christmas ceremonies which most likely came from a pagan tradition for Yule celebration.