The Sixth Doctor described the astral plane as a "collective illusion." The mind travelled out of its body via astral projection into the "universe's subconscious," dimensions of etheric mental energy that mirrored the physical world. It was notoriously tricky and beguiling, and took great will power to return from. (AUDIO: Point of Entry) He claimed that there was no such thing as time on the astral plane. (TV: The Two Doctors)
For several decades before 1996, the Great Intelligence was trapped in the astral plane through the Second Doctor's actions, only escaping briefly by destroying its own locus. (HOMEVID: Downtime, PROSE: Downtime)
Alistair Gordon Lethbridge-Stewart entered the astral plane within his dreams where he met Daniel Hinton who warned him to trust no one, because a lot of people were under the influence of the Great Intelligence. The astral plane was the only place in which Hinton was free from the Great Intelligence's influence, travelling there by using black magic. (HOMEVID: Downtime)