The first few Google results I found:
- Oxford Languages defines "hallucination" as "an experience involving the apparent perception of something not present."
- Merriam-Webster defines it as "a sensory perception (such as a visual image or a sound) that occurs in the absence of an actual external stimulus and usually arises from neurological disturbance (such as that associated with delirium tremens, schizophrenia, Parkinson's disease, or narcolepsy) or in response to drugs (such as LSD or phencyclidine)."
- Dictionary.com defines it as "a sensory experience of something that does not exist outside the mind, caused by various physical and mental disorders, or by reaction to certain toxic substances, and usually manifested as visual or auditory images."
YMMV on whether Picard's experience can be defined as a hallucination under these three. 1) is a definite yes. 2) depends on whether you count an alien device messing with your brain as an actual external stimulus ... although I think that
should count as a neurological disturbance. 3) requires it to be caused by a physical or neurological
disorder, a definition that this situation doesn't meet. So, I'd say the answer is "maybe."
The other distinction is whether it's a hallucination
episode. I think that category requires it to be clear to the audience that the character is experiencing something that can't possibly happen in the context of the show. If the audience can't tell if it's "real" (in context) or not, it just falls under "mind screw". The answer could turn out to be a hallucination or something totally different.
I think that's true here. For most of the sequence, it's entirely possible that Picard has been tossed back in time or has been shunted to an alternate reality or some such thing. Only after it's over is it made clear that the sequence wasn't real. So, even if we agree that his experience counts as a hallucination, I don't think it counts as a hallucination episode.
And you know, I went into this post expecting to argue the opposite.
Edit: How many of the other examples in this thread meet the definition? IS this a valid definition? I could still be convinced back to the other side.