A hallucination was a psychological term referring to visual, auditory, or some other combination of senses giving the brain unreal information, essentially causing a being to perceive something that was not real, and that others could not see.
In 2267, DoctorMcCoy speculated that seeing individuals who seemed to be the fictional characters Alice and the White Rabbit may have been a hallucination. When Spock later asked CaptainKirk if these and other strange sights might have been hallucinations, Kirk replied that one of the oddly appearing individuals, a former acquaintance named Finnegan, had painfully struck him on the jaw, suggesting these were not hallucinations. Thereafter, however, McCoy tried to reassure YeomanTonia Barrows that they were and that hallucinations were incapable of doing them any harm. As it turned out, the individuals were actually robots. (TOS: "Shore Leave")
In the final draft script of "Shore Leave", McCoy instead adamantly remarked about having seen Alice and the White Rabbit, "It was no hallucination." Also, a later scene description in the script noted that any filming method, such as slow motion, which might suggest a hallucinatory explanation to this and similar other odd sights in the episode should not be used, in favor of believability and "a logical, mechanical explanation of everything that happens to our people in this story."