Both "a hallucination" and "an hallucination" are used in English, but the more common and widely accepted usage is "a hallucination."
The choice between "a" and "an" before a noun depends on the sound that follows the article. If the noun begins with a consonant sound, you use "a." If it starts with a vowel sound, you use "an."
In the case of "hallucination," the initial "h" is often pronounced, so the word begins with a consonant sound. Therefore, "a hallucination" is the more common and grammatically correct form. However, in some dialects or regional accents, the "h" in "hallucination" may be silent, leading to the usage of "an hallucination." This usage is considered less standard and less common.
For the majority of English speakers, using "a hallucination" is the preferred and standard way to refer to the concept.