If you are addressing a middle age female, and you do not know her name, in South East Asia, Singapore, or Malaysia, the spelling is Auntie. In some North American Native cultures the use of the name "Auntie" for any woman older than you (usually between the ages 20 to 50) is a sign of respect and affection.
If you are addressing the sister of your parent, then it is a slang term. The proper word to use would be aunt. If you choose to use the slang, both spellings are listed in the Encarta World English Dictionary.
auntie or aunty can mean
The word auntie or aunty can mean:- A pet form for the word "aunt" An informal name for The British Broadcasting Corporation (otherwise known as the BBC) An informal name for the Australian Broadcasting Corporation (otherwise known as the ABC)