Blavatsky
Original sense
Reality is sevenfold: a stack of states of matter-consciousness, from the physical up to the atmic. A ‘plane’ is not a country you fly to; it is a degree of density of the one substance. She used the idea with more caution than the later, neatly labelled diagrams of Besant and Leadbeater.
The universe is worked and guided from within outwards… each plane is a state, not a locality.
Correspondence is the key: the seven principles in man mirror the seven planes in the cosmos. Later Theosophy named sub-planes and drew maps that New Age astral-travel manuals still copy.