Yggdrasil

The Yggdrasil is a figurative tree used to describe the nine realms by Asgardians. Each branch connects to one of the realms.

Thor
To explain Jane Foster the concept of the Nine Realms Thor draws the Yggdrasil Tree in a sketch pad.

Captain America: The First Avenger
After being hidden for many years the Tesseract was hidden in the wall with a carving of the Yggdrasil Tree on it.