Advertisement, the promotion of an item, service, company, and/or idea

Alzheimer's disease, a disease leading to the degeneration of neural function and dementia, usually seen in older people

Autonomic Dysreflexia, a condition in which "unreachable" parts of the nervous system (for instance, those beneath a damaged spine) malfunction

"at", "to", or "for", in some Latin phrases (such as ad lib or ad nauseam)

American Dad!, a Fox Network Animated Program

Andorra, a small country in the Pyrenees

Arrested Development (TV Series), a Fox Network TV program

Analog-to-digital converter (A/D), a circuit which converts continuous analog signals to discrete digital values

Air defence, anti-aircraft weaponry and systems

Adrian Peterson, football player, for "All Day"

Adenovirus, viruses of the family Adenoviridae.

Active Directory, a Microsoft administration program

Andorra's ISO 3166-1 country code

Academic Decathlon, a United States high school based academic competition

Associate's Degree, an academic degree

Aggregate demand, the total demand curve of an entire economy that is the sum of each individual's demand

Anno Domini, Latin for "In the Year of (Our) Lord", applied to years in the Julian and Gregorian calendars following 1 BC

Automatic differentiation, a way to speedily compute derivatives in computer programming

Assistant director, a film crew member who manages various tasks

Axiom of determinacy, a set theory axiom inconsistent with the Axiom of Choice

Adalius Thomas, football player

Autodynamics, an alternate to special relativity proposed by Ricardo Carezani in physics

Accidental discharge, the mistaken firing of a firearm

Athletic director, the administrator of the athletics program, usually at an educational institution

Aad (Omani tribe), a destroyed civilization mentioned in the Quran

Antonio Davis, basketball player

Ad, columnist in the English newspaper, The Saturday Sport (see also International Fanny Magnet)

.ad, Andorra's ccTLD (Internet Top Level Domain)

AD (band), a Christian rock band from 1984-1988

Active Disassembly, a technology involving specially designed building materials and components which are sturdy yet can easily be decomposed

Art director, the administrator in charge of art

Democratic Alternative (Malta) (Maltese: Alternattiva Demokratika), the Maltese Green party

Democratic Action (Spanish: Acción Democrática), a Venezuelan social democratic party

Audio description, a narration track for blind and visually impaired viewers of media such as TV and film

Atypical depression, a depression which makes you unable to express positive moods.

Action Directe (terrorist group), a French left-wing terrorist group

Analog Devices, a multinational producer of semiconductor devices.

Anno Diocletiani, the Era of (Emperor) Diocletian, an alternate year-numbering start date

Air Paradise (IATA code: AD), a defunct Indonesian airline

Atlantic & Danville (AAR reporting mark: AD), a defunct railway acquired by the (also now defunct/merged) Norfolk, Franklin and Danville Railway

Airworthiness Directive, requirements on legal usage of airplanes given by the FAA

Algemeen Dagblad, a Dutch newspaper

Anno Domini (gallery), a contemporary art gallery in San Jose, California

Ad Tak, Dutch cyclist

Active duty, a status of full duty or service, usually referencing full service in the armed forces

Destroyer Tender (US Navy Hull Classification: AD), support ships to aid the operation of a fleet of destroyers

The Air Department, part of the British Admiralty

Administrative domain, a collection of hosts and routers into one network

AD Music, a record label