XXused when the following word begins with a vowel sound [↪ a]:
➔ see usage noteaWORD CHOICE: WORD CHOICE: a, an Before a word beginning with a vowel sound, use an• an elephant • an umbrella • an obvious mistake!! Use anbefore an 'h' that is not pronounced• an hour later• an honest explanation!! Use abefore a 'u' that is pronounced like 'you'• a university • a unique opportunity!! Use anbefore an abbreviation that is pronounced with a vowel sound at the start• an SOS call• an MP3 file ➔ See alsoa
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.