An illusion was a false image.
Ian Chesterton and Barbara Wright initially believed that the dimensionally transcendental nature of the Doctor's TARDIS was an illusion. (TV: "An Unearthly Child")
The Morpho created the illusion that the city of Morphoton was luxurious when it was really filthy and squalid. (TV: "The Velvet Web")
The Spidrons kept their homeworld hidden behind the illusion of a supernova. (COMIC: Nova)
The Great City of the Exxilons created illusions for the Third Doctor and Bellal. (TV: Death to the Daleks)
Kalid used the Xeraphin to create illusions of Adric, the Melkur and a Terileptil in an attempt to frighten Nyssa and Tegan away from the Inner Sanctum. (TV: Time-Flight)
In the Dark Tower, the Second Doctor encountered illusions of Jamie McCrimmon and Zoe Heriot, while the Third Doctor encountered illusions of Liz Shaw and Mike Yates. (TV: The Five Doctors)
The Funhouse created an illusion of Peri Brown for the Sixth Doctor and Frobisher. (COMIC: Funhouse)
Qataka created the illusion of a white antelope to lure Gilgamesh to her. (PROSE: Timewyrm: Genesys)
The Doctor's TARDIS created surreal illusions which only the Seventh Doctor and Ace could see to warn them that a creature was trying to break into it. (PROSE: Cat's Cradle: Time's Crucible)
Charles Dickens initially believed that the Gelth were illusions. (TV: The Unquiet Dead)
In 1982, giant cockroaches trapped a group of people in a bunker by creating the illusion that a nuclear war had broken out. (AUDIO: Day of the Cockroach)
Kera-Bera Beasts created illusions to become close to individuals, then feed off their life energy. (COMIC: The Stray)
Bilis Manger created the illusions of Lisa Hallett, Diane Holmes and Toshiko Sato's mother to convince members of Torchwood to open the rift. (TV: End of Days)