As Laura mentioned, it's "an". However, it's not that the word begins with a vowel, but that it begins with a vowel sound. For example, the word "honor" takes "an", as in "It's an honor to meet you". This is because "honor" begins with a vowel sound, even though its first letter is a consonant. Another example would be "I'm attending a university function this evening". Here, even though the word "university" begins with a vowel, it takes "a" as "university" begins with a consonant sound.