what does A D mean?

Answer 1

(You just got usurped, Mr. Top Contributor!!!)

AD is an acronym that can mean any of the following:

AD Adapted
AD Administration
AD Adult
AD Advertisement
AD Alternate Days
AD Anno Domini (Latin: In the Year of Our Lord; referring to the Gregorian calendar system)
AD Archduke
AD Absolute Deviation
AD Academic Decathlon
AD Academic Don
AD Access Denied
AD Accession Document
AD Accidental Damage
AD Accidental Death (life insurance)
AD Accidental Discharge
AD Acción Democrática (Democratic Action Party)
AD Accumulated Depreciation (accounting)
AD Accumulator Dehydrator (automotive A/C component)
AD Ace Deuce (band)
AD Acknowledgement Due (postal)
AD Acoustic Deficiency
AD Acoustic Densitometry (ultrasound)
AD Acquisition Directorate
AD Acquisition Doppler
AD Acronym Dictionary
AD Activation/Deactivation
AD Active Delivery
AD Active Directory
AD Active Duty
AD Activity Director
AD Activity Discard (ITU-T)
AD Activity Driver
AD Adaptive Control (welding)
AD Adaptive Development
AD Add Delegations (security permissions)
AD Addendum
AD Addendum Document (to an OSI Standard; ISO)
AD Address Data
AD Adellion (game)
AD Adjoint (mathematics; group theory)
AD Administrative Distance (routing information source rating)
AD Administrative Domain
AD Admiral
AD Adopted Daughter
AD Adrian Peterson (Oklahoma Sooners football player)
AD Advance Design
AD Advance Development
AD Advanced Deployability
AD Advanced Design
AD Advanced Development
AD Advo, Inc (stock ticker symbol)
AD Affinity Diagram (Sprint - Quality)
AD After Date
AD After Death
AD After Deployment
AD After Dianetics (L. Ron Hubbard)
AD After Divestiture
AD Aggregate Demand (economics)
AD Agility Dog (canine agility title)
AD Agnus Dei
AD Aid to the Disabled
AD Air Defense
AD Air Directive
AD Air Division
AD Air Dried (timber)
AD Aircraft Division (USN)
AD Airdrop
AD Airspace Deconfliction
AD Airworthiness Directive
AD Akabane Disease
AD Alarm Data (Alcatel)
AD Albus Dumbledore (Harry Potter)
AD Alcohol Deprived
AD Alexis Denisof (actor)
AD Algemeen Dagblad
AD Alien Dice (webcomic)
AD Alliance for Democracy (Nigeria)
AD Allocation Descriptor (OSTA universal disk format)
AD alt.digitiser (newsgroup)
AD Alternating Direct
AD Alternating Directly
AD Alternativa Demokratika/Alliance for Social Justice (Malta)
AD Alzheimer's Disease
AD Amigos de Dios (Guatemala church support group)
AD Amplitude Distortion
AD An der Donau (German:on the Danube)
AD Anaerobic Digestion
AD Analog Devices, Inc.
AD Analog-to-Digital
AD Analysis of Dispersion
AD Ancillary Device
AD Anderson-Darling Statistical Test
AD Andes
AD Andorra (top-level domain)
AD Andrew Dick (manuscript surcharge mark, British East Africa)
AD Animal Development (genetics)
AD Ante Diem
AD Anti-Dumping
AD Antidepressant
AD Antidisturbance
AD Antidumping Duty
AD Antimatter Decelerator
AD Antodromic
AD Antonio Davis
AD Aperture Card Diazo
AD Aperture Door
AD Appellate Division
AD Applicable Document
AD Application Data
AD Application Developer
AD Application Development (IBM)
AD Application Directory
AD Approved Direction
AD Arbeitsgemeinschaft Druckbehälter
AD Architectural Design
AD Archive Device
AD Area Defense
AD Areal Density
AD Armament Division
AD Armored Division
AD Army Depot
AD Arrested Development (rap group)
AD Arrested Development (TV show)
AD Art Director
AD Artificial Dumbness
AD As Drawn
AD Asperger's Disorder
AD Assembly District
AD Assembly Drawings (Nortel)
AD Assignment Date
AD Assistant Director
AD Assistant Driller
AD Associate Degree
AD Associate Director
AD Associated Document
AD Athletic Director
AD Atmospheric Deposition
AD Atopic Dermatitis
AD Attack Director
AD Attention Danger (gaming clan)
AD Attitude Determination
AD Audio Drama
AD Außer Dienst (German: Retired)
AD Authorized Dealer
AD Authorized Delay
AD Authorized Distributor
AD Auto Disable (syringe)
AD autodestruct (syringe)
AD Autograph Document
AD Automated Data
AD Automatic Damper
AD Automatic Depressurization
AD Automatic Differentiation
AD automatic distribution (US DoD)
AD Autonomous Domain
AD Autosomal Dominant (genetics; dominant gene on one of the 23 pairs of autosomes)
AD Auxiliary Destroyer Tender
AD Auxiliary Drain
AD Average Deviation
AD Aviation Division
AD Aviation Machinist Mate (US Navy rating)
AD Awaiting Disconnection
AD Azure Dreams (role-playing game)
AD Destroyer Tender (US Navy ship designation)
AD Partia Aleanca Demokratike (Party of the Democratic Alliance, Albania, aka: PADSh)
AD priority add-on (US DoD)
AD Right Ear

Answer 2

Yeah, what everyone else said, especially the first answer, is right. Basically, it normally means Anno-Domini, like the first one said, and is sometimes called After Death. But the most common use is to distinguish the B.C. period from A.D., which transitioned when Christ died.

Answer 3

Anno Domini (Latin: In the Year of Our Lord; referring to the Gregorian calendar system)

Answer 4

attention deficit