The mystic nature of a mage & their Avatar. Essence is supposed to influence Seekings and the mage’s ultimate destiny. It manifests as:
There is a postulated 5th Essence called Infinite. In the first 2 editions, the Avatar was a part of the mage. Revised Edition & Mage20 separated the Avatar from the mage so the Essence became a quality of just the Avatar. Called the Avatar’s personality by some Mage writers but that is an oversimplification. Called Eidolon by Technocrats.
Initiates of the Art has info on Essences in chapter 2.
pg. 141 1st Ed core said of Dynamic Essence “This is by far the most common Essence.” This statement is repeated on pg. 114 of 2nd Ed core. Pg. 42 of Initiates of the Art says Questing Essence is common among Tradition mages.