Akasha is the Sanskrit word meaning space or sky. In Hinduism it is one of the five elements (panchamahabhoota), the others being prthivi 'earth', apa 'water', agni 'fire' and vayu 'air'. In paganism and wicca, it is the unifying energy that is inherent in every living creature on the planet, and in all of the 4 elements (earth, air, water, and fire) that make up the natural world as we know it. On the pentagram, akasha is assigned to the fifth top point, and the "other" element. Essentially it is spirit: the human spirit and soul, and the all-encompassing spirit and energy of the Goddess and God.

Also called the quintessence, aether, or sacred sound.

See also: Akashic Records.

References

Indian Physics Outline of Early History, S. Kak (2003)

EDIT