Astral projection

Astral projection was a psychic power that involved the mind (or soul) leaving the physical body and entering the astral plane. Humans developed a method of astral projection which allowed for psychically travelling to different planets as opposed to doing so in starships. (TV: Image of the Fendahl)

The Sixth Doctor described the astral plane as a "collective illusion." The mind travelled out of its body 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)

The Fendahl had such enormous energies that it was able to project itself from the Fifth Planet to Earth before being trapped in a time loop, possibly ravaging Mars along the way. (TV: Image of the Fendahl)

The Osirans were capable of mentally projecting themselves, and also designed psi-projectors which let them travel through space using only the power of their minds. (TV: Pyramids of Mars, PROSE: The Sands of Time)

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. There 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 Daniel Hinton was free from the Great Intelligence's influence. He went there by using black magic. (HOMEVID: Downtime)