AD or ad may stand for:

• ad or advertisement, see advertising
• ad- prefix
• Administrative domain
• Air Defence
• Andorra, ISO 3166-1 alpha-2 country code
• Anno Domini ("In the Year of [Our] Lord"). This year is A.D. 2005.
• anni Diocletiani
• Assistant Director
• Destroyer Tender , US Navy hull classification symbol
• Alzheimer's disease
• The Air Department of the British Admiralty
• The word ad is Latin for "at" or "to" in some phrases taken from Latin.
• Norfolk, Franklin and Danville Railway (AAR reporting mark AD)
• Microsoft's Active Directory
• is the IATA code for Air Paradise
• In economics, AD is usually used to represent aggregate demand or aggregate expenditure.
• Artium Doctor (Doctor of Arts)