The mantric syllable HUM in the context of the three fold OM AH HUM represents mind. It is visualized at the heart. Unlike our western tradition where we point to our heads when we think of the mind, for the Tibetans the most subtle mind resides in the heart.

In the context of the mantra OM AH HUM, it represents the mind. . It contains the idea of the transformation of our ignorant mind into a pure wisdom mind. One that is freed from habits, both aquired and innate, that cause suffering.

"Purity must be achieved by an indivisible unity of method and wisdom, symbolized by the final syllable hum, which indicates indivisibility. According to the sutra system, this indivisibility of method and wisdom refers to wisdom affected by method and method affected by wisdom. In the mantra, or tantric, vehicle, it refers to one consciousness in which there is the full form of both wisdom and method as one undifferentiable entity. In terms of the seed syllables of the five Conqueror Buddhas, hum is the seed syllable of Akshobhya - the immovable, the unfluctuating, that which cannot be disturbed by anything." Dalai Lama

The syllable HUM is used in meditation practices that involve imaginatively recreating the dissolution of the elements at the time of death.