In simple terms, a hallucination is the perception of something that is not actually there. Medical literature defines a hallucination as a perception without a corresponding stimulus.
Hallucination can takes form of sounds or voices, smell, taste, vision, or even physical sensation. Visual Hallucination can be as simple as sensing movement in your peripheral visions or as complex as seeing a piece of furniture move or a group of people dancing.
The two famous examples of phenomenon that mimic hallucinations are Purkinje Lights and The Gandzfeld Procedure.