Hebrew, “Foundation.” Yesod is the foundation God has built the world upon. Because of this, it serves as a transmitter between worlds; the sephirot above, and the mundane reality below. It is placed in the center of the Tree, light of the upper sephirot gather in Yesod and are channeled to Malkuth below through one’s third eye.
Yesod is associated with the sexual organs, the part of the body given the power of creation. While the floods drown in Noah’s time, so too did they bring life back into the world. Yesod collects the vital forces of the sephirot above, and transmits them to the world below. Though this, the earth is able to interact with the divine.
See Book of Remembrance 29:12, 30:28-30.