Which is grammatically correct: An hallucination or a hallucination?

Update: I question because I know the rule but in the best selling book 'Let Me In' they use an hallucination (page 392) and I thought maybe there was a special rule I didn't know or sometimes there are two schools of thought. Now it is a translated book from Swedish but I still am surprise at a mistake in a major... show more I question because I know the rule but in the best selling book 'Let Me In' they use an hallucination (page 392) and I thought maybe there was a special rule I didn't know or sometimes there are two schools of thought. Now it is a translated book from Swedish but I still am surprise at a mistake in a major published book.
thanks for the answers...you guys are great
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