According to the Oxford American English Dictionary it means Anno Domini, as zAk mentioned. Wallahu 'alam.

An interesting grammar note the dictionary had was the following:

USAGE The abbreviation for Anno Domini, AD , typically is written in small capitals and should be placed before the numerals, as in: AD 375 (not | 375 AD). However, when the date is spelled out, it is normal to write, for example, | the third century AD (not | AD the third century). The abbreviation BC (before Christ) appears after the date: | Plato was born in 427 BC —that is to say, in the fifth century BC. In recent years, some writers have begun using the abbreviations CE (of the Common Era) in place of AD, and BCE (before the Common Era) in place of BC, in consideration of a more secular and international readership.

The part about AD coming before the number was new to me.

Coming back to your original question, br. wkas, I think it is prefersble for Muslims to use CE and BCE (as oppose to AD and BC) when referring to the gregorian calendar.