AB, Ab, or ab may refer to:

• American Bandstand, a music-performance television show
• Analecta Bollandiana, an academic journal
• Ancienne Belgique, a concert hall in Brussels, Belgium

• Akcinė bendrovė, Lithuanian equivalent of an S.A. corporation
• Aktiebolag, Swedish for "corporation", similar to AG, Ltd or Inc

• A & B High Performance Firearms, a defunct sporting firearms manufacturer
• AB Airlines, a defunct British airline
• AB Groupe, a French broadcasting group
• Activision Blizzard, American holding company for Activision and Blizzard Entertainment
• Air Berlin (former IATA airline code AB), a former airline operating 1978–2017
• Alderson-Broaddus College, a liberal-arts college in West Virginia, US
• Alfa-Beta Vassilopoulos, a Greek supermarket chain
• Allen-Bradley, a brand of industrial control products, manufactured by Rockwell Automation
• AllianceBernstein (New York Stock Exchange Symbol AB), a US-based asset management firm
• American Biograph (film logo AB), a former motion picture company
• Anheuser-Busch, a brewing company
• Bonza (IATA code AB), an Australian airline

• Ab (cuneiform), a written syllable
• Ab (Semitic), a Semitic word for "father"
• ab, a Welsh patronymic
• Ap (water), Vedic Sanskti term for water, surviving as In Persian as Āb

• Able Seaman (occupation), a civilian occupation
• Able Seaman (rank), a naval rank
• Airman Basic, the lowest rank in the United States Air Force

• Afrikaner Broederbond, a South African secret society from 1918 to 1994
• Aryan Brotherhood, an American gang

• Anzab-e Olya or Āb, a village in Iran
• AB postcode area, UK, including Aberdeen, Scotland
• Albania (WMO country code)
• Alberta (Canadian Province/Territory Code)
• Air Base, used by the United States Air Force for overseas bases
• Aschaffenburg (vehicle registration plate), Germany

• AB (star catalogue) (Azzopardi / Breysacher), a catalogue of Wolf Rayet stars in the Small Magellanic Cloud
• Aggregate base
• AB Class amplifier, a classification of electric amplifier
• Category of abelian groups (Ab), in mathematics
• ApacheBench, a command line tool for Apache HTTP Server

• Alabamine, a former name of the element astatine
• Albite, a feldspar mineral
• Ammonia borane, a chemical compound
• Antibonding (a.b.), used to describe the character of orbitals in atomic and molecular electronic structure

• AB blood, a blood type in the ABO blood group system
• AB toxin, Type III toxin secreted by some pathogenic bacteria
• Antibody (medical abbreviation)
• Rectus abdominis muscle (common name abs), a paired stomach muscle

• Ab (Egyptian heart-soul concept), a concept of the heart-soul in ancient Egyptian religion
• Ab., a Hebrew abbreviation related to Pirkei Avot, a compilation of teachings and maxims from the Mishnaic period
• Aitareya Brahmana, an ancient Indian collection of sacred hymns

• Akademisk Boldklub, a Danish professional football club
• Argja Bóltfelag, a Faroese association football club
• Aviron Bayonnais, French rugby union club
• At bat, in baseball statistics

• NZR Ab class, a New Zealand steam locomotive
• Armed boarding steamer, used by the UK during World War I for boarding enemy vessels
• AB, a US Navy hull classification symbol: Crane ship (AB)

• AB de Villiers (born 1984), South African middle order batsman and wicket-keeper
• Adrien Broner (born 1989), American Boxer, nicknamed AB
• Allan Border (born 1955), Australian cricketer, nicknamed A.B.
• Antonio Brown (born 1988), American football player, nicknamed AB

• Ab (given name), a short form of Albert (or occasionally Abraham or Abbott)
• Ab (Mandaean month), a month of the Mandaean calendar
• AB, a South Australian dish made of hot potato chips, gyro meat and sauces, similar to a Halal snack pack
• Abkhaz language (ISO 639-1 language code ab), a Northwest Caucasian language spoken mostly by the Abkhaz people
• Adult baby, a person who practices paraphilic infantilism
• Bachelor of Arts (Latin: Artium baccalaureus)
• Assembly Bill, a type of legislation in the US
• AB (game), a guessing game similar to Mastermind

• A/B (disambiguation)
• BA (disambiguation)
• A-flat (disambiguation) ("A♭"), the musical note, scale, or key
• Av (month), a month in the Hebrew calendar
• Amyloid beta (Aβ), peptides of amino acids that are involved in Alzheimer's disease