Major Yin (太阴) is one of the Four Phenomena in Chinese metaphysics. It stems from the principle of Yin and Yang, where neither can exist without the other. Major Yin represents the largest portion of Yin energy, with Minor Yang co-existing within. It is associated with the Black Tortoise, the North direction, and the season of Winter. This concept is essential for understanding the dynamic balance and interactions of energies in the natural world.