Ah" is an interjection.

Ah!" is an interjection.

Inspector Gregory was full of his case, and poured out a stream of remarks, while Holmes threw in an occasional question or interjection.

In English there are eight main parts of speech: noun, verb, adjective, adverb, pronoun, preposition, conjunction and finally interjection.

Nouns, pronouns, verbs, adjectives, adverbs, articles, prepositions, conjunctions, and interjections are the parts of speech in English.