ah

from The American Heritage® Dictionary of the English Language, 5th Edition.

• interjection Used to express various emotions, such as satisfaction, surprise, delight, dislike, or pain.

from The Century Dictionary.

• An exclamation expressive of pain, surprise, pity, compassion, complaint, contempt, dislike, joy, exultation, etc., according to the manner of utterance.

from the GNU version of the Collaborative International Dictionary of English.

• interjection An exclamation, expressive of surprise, pity, complaint, entreaty, contempt, threatening, delight, triumph, etc., according to the manner of utterance.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• interjection An expression of relief, relaxation, comfort, confusion, understanding, wonder, awe, et cetera according to uttered inflection.
• interjection A word used for drama or emphasis.
• interjection A syllable used to fill space, particularly in music.
• noun An instance of the interjection ah.
• pronoun Eye dialect spelling of I., most often indicating that the speaker is using an American (particularly Southern) accent.