An hallucination was the perception of something that was not present.
After Field Major Styre captured Sarah Jane Smith, he subjected her to Experiment 7: resistance to fear. He put a device on her head which gave her hallucinations. (TV: The Sontaran Experiment)
After landing on Varos, a guard thought the TARDIS was an hallucination caused by the Punishment Dome. The Sixth Doctor later had an hallucination of being in a desert which caused him to believe he was dying of thirst. (TV: Vengeance on Varos)
The psychedelic sky and landscape of Simia KK98 caused most humanoids to experience hallucinations. (PROSE: Dead Romance)
Just before regenerating, the Eleventh Doctor had an hallucination of the young and adult Amy Pond. (TV: The Time of the Doctor)