The definition of ah is a way to express emotions such as surprise, fear and disgust.(interjection)
An example of ah is what someone might say when a ghost pops up in a haunted house.
See ah in Webster's New World College Dictionary
Origin: natural exclamation similar to Fr & L ah, Gr a, ON æ, OHG a, Sans ā
Origin: L Anno Hegiraein the year of the Hegira
See ah in American Heritage Dictionary 4
Learn more about ah