- A ... A-V node
- A. H. Robins Company ... Aarhus
- Aarhus University ... abatement
- Abauzit, Firmin ... Abbott, Robert Sengstacke
- Abbott, Scott ... ʿAbd Allāh ibn Thunayan
- ʿAbd Allāh ibn Yasīn ... Abdullah, Farouq
- Abdullah, Omar ... Aberewa
- Aberfan ... Abiola, Moshood Kashimawo Olawale
- abiotic environment ... abortion rights movement
- abortion, epizootic ... Abrikosov, Alexey A.
- Abrikosov, Alexey Alexeevich ... absorption
- absorption ... Abū al-ʿAbbās al-Walīd ibn ʿAbd al-Malik ibn Marwān
- Abū al-ʿAbbās Muḥammad ibn Yazīd ... Abū Saʿīd
- Abū Saʿīd ... Abundance of Katherines, An
- abundance of the elements ... Académie Royale
- Académie Royale de Danse ... Acarina
- Acarine disease ... Acción Republicana
- accipiter ... Acela
- Acela Express ... Achaia
- Achaios ... achromatic point
- Achromatium ... acne vulgaris
- ACO ... acquired immune deficiency syndrome
- acquired immunity ... acrylic compound
- acrylic fibre ... Actinostachys
- Actinulida ... actor-network theory
- Actors Studio, The ... ad hominem
- ad ignorantiam ... Adam’s Peak
- Adam’s Rib ... Adams-Stokes syndrome
- adamsite ... adding-up game
- Addington, Henry, 1st Viscount Sidmouth of Sidmouth ... Adelphi
- Adelphi ... adhyatmavidya
- adhyatmika ... Adler, Steve
- Adler, Alfred ... ADNOC
- Ado ... adrenal crisis
- adrenal gland ... Adur, River
- Ādur-Anāhīd ... Adventures of Tom Bombadil and Other Verses from the Red Book, The
- Adventures of Tom Sawyer, The ... AEEU
- AEF ... Aeolis
- Aeolopithecus ... aerostatic craft
- aerostatics ... Afakani language
- Afanasev, Aleksandr Nikolayevich ... Afghan Revolutionary Council
- Afghan War ... African Blood Brotherhood
- African Blood Brotherhood for African Liberation and Redemption ... African religions
- African river martin ... AFSCME
- Afshārid Turkmen ... Agadir
- Agadir ... Agate, James
- Agate, James Evershed ... Agence spatiale canadienne
- Agence Spatiale Européenne ... aging
- aging ... agon
- agonía del cristianismo, La ... Agricultural College of Utah
- Agricultural College of West Virginia ... Aguascalientes
- Aguascalientes, Autonomous University of ... Ahenobarbus, Lucius Domitius
- Aḥer ... Aḥmed Grāñ
- Ahmed Haşim ... Aichi
- Aichi Biodiversity Targets ... Aims of Interpretation, The
- Aimwell School ... Air Multiplier bladeless fan
- Air New Zealand flight TE901 disaster ... AIRFA
- airfoil ... Aizawa Seishisai
- Aizawa Yasushi ... Akaler Sandhane
- Akali ... Akhmadulina, Bella
- Akhmadulina, Izabella Akhatovna ... Aksakov, Ivan Sergeevich
- Aksakov, Ivan Sergeyevich ... Al-Quds University
- al-Thaʿalibī, ʿAbd al-Azīz ... Alamo
- Alamo Bowl ... Alatri
- Alatrium ... Albany Congress
- Albany Evening Journal ... Albert of Lauingen
- Albert of Mainz ... albite twin
- albite-epidote-hornfels facies ... Alcamenes
- Alcamo ... Alcock, Sir John William
- Alcock, W. J. ... Aldrich, Bess Genevra Streeter
- Aldrich, Lyman T. ... Alençon
- Alençon lace ... Alexander II
- Alexander III ... Alexandria Municipal Museum
- Alexandria Protocol ... Alfonso el Emperador
- Alfonso el Franco ... algebraic surface
- algebraic topology ... ʿAlī Ḥasan al-Majīd
- ʿAlī I ibn Mazyad ... alienation
- alienation effect ... alkaline phosphatase
- alkaline plutonic rock ... All My Sons
- All Nations ... All-Russian Congress of Soviets
- All-Russian Extraordinary Commission for Combatting Counterrevolution and Sabotage ... allelic heterogeneity
- allelic homogeneity ... Alles, Gordon
- alley cat ... Allison, Dennis
- Allison, Fran ... allyott
- Allyson, June ... almuce
- Almudena Cathedral ... Alpenrhein River
- Alpensinfonie, Eine ... Alphabetical Order
- Alphabetland ... Alster River
- Alston, Barbara ... alternative education
- alternative energy ... Alucitidae
- Alucitoidea ... Alver, Amalie
- Alverio, Rosa Dolores ... Amalasuntha
- Amalekites ... Amaranthus cruentus
- Amaranthus graecizans ... Amazing Spider-Man, The
- Amazing Spider-Man, The ... Ambikā-Mātā
- Ambikapur ... Amdang language
- AMDG ... America Again: Re-becoming the Greatness We Never Weren’t
- America and the World War ... American bighorn sheep
- American Birth Control League ... American Enterprise Institute
- American Entomology ... American Indian Arts, Institute of
- American Indian boarding school ... American oil palm
- American organ ... American Scholar, The
- American School Citizenship League ... American whitewood
- American wigeon ... Amfípolis
- Ámfissa ... Amirante Isles
- ʿĀmirids ... Ammonoidea
- Ammons, A. R. ... Amorous Fiammetta
- Amorous Quarrel, The ... Amphioxus
- amphioxuses ... Amrouche, Marguerite Taos
- Amrouche, Marie-Louise ... amygdaloid body
- Amygdaloideae ... Anaconda Company
- Anaconda Copper Mining Company ... analogue modulation
- analogue signal modulation ... Ananta
- Ananta ... Anastasya Zakharina-Yureva
- anastatic printing ... Ancestry.com LLC
- Anche le donne hanno perso la guerra ... And Just Like That…
- …And Justice for All ... Anderson, Benedict
- Anderson, Benedict Richard O’Gorman ... Andhradesha
- Andi languages ... Andreus, Antonius
- Andrew Albert Christian Edward, duke of York, earl of Inverness, and Baron Killyleagh ... Andronicos, Manolis
- Andronicus Cyrrhestes ... anesthesia
- anesthesiology ... Angélique, Pierre
- Angell, James Burrill ... angle of incidence
- angle of polarization ... Anglo-Portuguese Convention
- Anglo-Russian Convention ... angular momentum quantum number
- angular momentum, conservation of ... Animal Ecology
- Animal Ecology and Evolution ... anitya
- Anius ... Annales rerum Anglicarum et Hibernicarum Regnante Elizabetha
- Annales school ... annihilation radiation
- Anning, Mary ... Anodorhynchus glaucus
- Anodorhynchus hyacinthinus ... Anou, Chou
- Anouilh, Jean ... Anson, Pop
- Ansongo ... Ante-Nicene Father
- anteanaresis ... Anthocerotidae
- Anthocerotophyta ... anthroposophy
- Anthrosol ... anticancer antibiotic
- anticancer drug ... antiknock agent
- antiknock rating ... Antipater
- Antipater ... antiviral drug
- antivirus software ... Antropología, Museo Nacional de
- Antropov, Aleksey ... Anyang
- Anyang Art Park ... AP
- Ap ... apella
- Apelles ... Apia
- Apiaceae ... Apodi
- apodictic law ... Apology
- Apology Against the Arians ... apotropaic power
- Apoxyomenos ... Appenzell Rhodes-Intérieures
- apperception ... Appointment in Samarra
- Appointment with Danger ... ’Apu Mayta
- Apu-Punchau ... Aquaviva, Claudio
- aqueduct ... Arab Socialist Renaissance Party
- Arab Socialist Republic ... Arachnocampa luminosa
- arachnodactyly ... Aranda, Rafael
- Aranda, Rafael; Pigem, Carme; and Vilalta, Ramon ... Arbeitergilden der Gegenwart, Die
- Arbeitsgemeinschaft der Öffentlich-Rechtlichen Rundfunkanstalten Deutschlands ... Arcaro, George Edward
- Arce Catacora, Luis Alberto ... Archelon
- archenteron ... Architeuthis dux
- architrave ... Arcueil circle
- Arculf ... area, culture
- Area, the ... Arezzo
- Arezzo, Guittone d’ ... Argishti II
- Argo ... Ariadne auf Naxos
- Ariadne on Naxos ... Arisaema triphyllum
- Arise, My Love ... Arizona Republic, The
- Arizona Science Center ... arm-and-body
- Arm-in-Arm Convention ... Armenian National Movement
- Armenian oak ... Armstrong, Billie Joe
- Armstrong, David Malet ... Arno River
- Arno, Fiume ... Arosa
- Arosa, Gustave ... Arretium
- Arrhenatherum ... Arseneva, Natalla
- arsenic ... Art Museum of Estonia
- Art Museum of Romania ... Artaxerxes I
- Artaxerxes I Macrocheir ... Arthrodiriformes
- arthrogryposis ... artificial organ
- artificial porcelain ... Artykuly Henrykowskie
- Artyom ... Aryeh, Judah
- aryepiglottic fold ... Asamando
- Asamkirche ... ASCI
- asci ... Ash Wednesday
- Ash Wednesday ... Ashkenazy, Vladimir
- Ashkenazy, Vladimir Davidovich ... Asia
- Asia ... ʿAsīr
- Asirgarh ... aspect
- aspect ratio ... Assal, Lake
- Assam ... assimilation efficiency
- assimilation model ... Assur-nasir-apli I
- Assur-nasir-apli II ... astome
- Aston Martin ... astronomical observatory
- Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac ... At Large
- At Long Last Love ... Ataulphus
- Atauro ... Atheistic Humanism
- Athel tree ... Athos, Mount
- Athrotaxis ... Atlantic lobster
- Atlantic lowland ... atmospheric divergence
- atmospheric electricity ... atomic weight
- Atomico Ventures ... Atsuta Shrine
- ATT ... Attikē
- Attike ... Aubert, Jean
- Auberval, Jean D’ ... auditory system
- auditory tube ... August III Wettin
- August Is a Wicked Month ... Aulococeras
- auloi ... auroch
- aurochs ... Austin, J. L.
- Austin, John ... Australian gold rushes
- Australian Government Employee Organisations, Council of ... Austrian National Library
- Austrian Netherlands ... autobiography
- Autobiography ... automatic train protection
- automatic transmission ... Autun, Council of
- autunite ... Avataṃsaka
- Avatamsaka-sutra ... Avery turbine
- Avery, Byllye ... AVLIS
- AVMT ... awdlau
- Awe, Loch ... axiom of pairing
- axiom of power set ... Ayer, Francis Wayland
- Ayer, Sir Alfred Jules ... Ayyām, Al-
- Ayyappan ... Azione, Squadre d’
- aziridine ... ʿĀʾishah bint Abī Bakr
- Aʿshā, al- ... aʿyān