It is definitely "a."The only time you use "an" before "h" is when the "h" is silent and thus the word sounds as though it starts with a vowel, which requires an "an." Example: It's an honor to be here. In that sentence, the "h" is silent. However, before Halloween, use "a" since the "h" is pronounced. Ex: I'm having a Halloween party.
I see that you have asked htodd to answer this but I couldn't resist. An object perceived during a hallucinatory episode. He refused to believe that the angel was a hallucination. My vote is also for "A"
I just had an interesting thought, at least to me anyway.
Victoria Lynn is correct in writing that the article "an" is used before words beginning with "h" when the "h" is silent. But I noticed that you (Rosemay) are from the UK and now living in New Zealand.
I believe you may pronounce more words with silent "h"s than Americans. For example, when Americans speak the word "hallucination" the "h" sound is heard, but perhaps for some people with a British accent the "h" may be silent (but I'm not sure). Therefore, if you speak with an accent where the "h" is silent in "hallucination" then it would be correct for you to use the article "an" in front of the word "hallucination."
But I see you asked the question of an international editor, htodd, so I would be interested to see her response. I am curious if the rules differ when writing for an international audience.
English language like many other languages is based on sound. Phonetics. Vowel sound is important. Since hallucination has consonant sound it will have 'a' before it. Had it been 'honesty' or 'hour' we would have put 'an' before it because they have a vowel sound.
an. Read it out loud to yourself when you are using it in a sentence and it should tell you straight away what is correct. I am Aussie so maybe that is why I disagree with most of you. An hallucination sounds better than a hallucination.