According to the Kalachakra teachings, (1) the subtlest of all particles, having only one sensory quality or property, namely sound. When a universe is bare, there are only space particles in it – according to different commentaries, either only one or many. Within them is a trace of the other four grosser elements, not just the potentials for them. These traces are not bound together, but rather are “fragmented.” With the formation of a universe, the winds of shared karma of those who will be reborn in that universe serve as the condition for the step-by-step arising of the grosser elements through the fusion of these traces. (2) The space between particles at the time particles disperse from sticking together.
Tibetan: ནམ་མཁའི་རྡུལ་ཕྲན། nam-mkha’i rdul-phran