Consciousness was being aware of oneself and made an individual or group able to feel.
After the Doctor escaped from the Timewyrm twice, the Timewyrm sent Anthony Rupert Hemmings' consciousness into the Doctor's mind. He took over the part of the Doctor's mind which was home to his third incarnation's personality, creating an idealised Nazi city. He imprisoned the Doctor and Ace when they arrived, but the Doctor and his past self sent a telepathic message to Saul, who worked with Emily Hutchings to draw the Hemmings' consciousness out of the Doctor's mind and back into his own severed head, killing him. (PROSE: Timewyrm: Revelation)
Clyde Langer's body was briefly taken over by the Eleventh Doctor's consciousness who wanted to contact Sarah Jane Smith and Jo Grant but was stuck at the wastelands of Crimson Heart. (TV: Death of the Doctor)
After their death, many humans' consciousnesses were taken by 3W and uploaded to the Nethersphere. Their consciousness was still connected to their body and made them feel pain. Seb gave them the opportunity to disconnect from their body turning them into Cybermen. Danny Pink was one of these humans. (TV: Dark Water)
The Tenth Doctor's consciousness had been accidentally transferred into the TARDIS' memory core. After reaching the data/matter converter the Doctor's consciousness could return back to his body. (GAME: Eye of the TARDIS)