- A ... A-20
- A-3 ... AACM
- AACR2 ... Abaco
- abacus ... Abbate, Niccolò dell’
- Abbati, Giuseppe ... ʿAbd al-Muʾmin
- ʿAbd al-Muʾmin ibn ʿAli ... abdominal cavity
- abdominal ectopic pregnancy ... Abela, Robert
- Abelam ... Abhidhamma Pitaka
- Abhidhammattha-sangaha ... ablution
- ABM ... Abraham Lincoln
- Abraham Lincoln Battalion ... Absaroka Sequence
- Abscam ... abstract reference
- abstract space ... Abu Dhabi National Oil Company
- Abū Dhahab ... Abū Yūsuf
- Abū Yūsuf Yaʿqūb ... Abuza, Sophie
- Abūʾ al-Khayr Khan ... Academy Bay
- Academy Curve ... ACC
- Acca sellowiana ... accismus
- Accius, Lucius ... Acentropus
- Acephala ... Achanak
- Achard, Franz Karl ... Achterberg, Gerrit
- Achterhoek ... ACNA
- acne ... acquired characteristics, inheritance of
- acquired contemplation ... Acrux
- acrylamide ... Actinophrys sol
- Actinopterygian ... Acton
- Acton of Aldenham, John Emerich Edward Dalberg Acton, 1st Baron, 8th Baronet ... Acyrthosiphon pisum
- Aczél, György ... Adam brothers
- Adam Dalgliesh ... Adams, Marian
- Adams, Maude ... Addams Family, The
- Addams Family, The ... Adelaide Symphony Orchestra
- Adélaïde, Sainte ... ADHA
- Adhaim ... Adiyozhukkukal
- Adja ... Admiral, the
- Admirals ... Adoration
- adoration ... Adso of Montier-en-Der
- adsorbed atom ... Adventure Consultants
- adventure game, electronic ... ʿAdwiyyah
- Ady, Endre ... Aelfheah, Saint
- Aelfled ... aerobe
- Aerobee ... Aesthetica
- Aestheticism ... affective valence
- affector ... Afonso IV
- Afonso o Africano ... African golden oriole
- African golden wolf ... African Unity, Organization of
- African vine snake ... After the Fire
- After the Gold Rush ... Against the Odds
- Against the Ropes ... agave nectar
- Agave sisalana ... Agent Running in the Field
- Agent W ... Agitator’s Notebook, The
- agitatsiya propaganda ... Agony of Christianity, The
- Agoondarro waa u nacab jacayl ... Agricultural Research Service
- agricultural revolution ... Aguilar v. Felton
- Aguilar, Grace ... Ahikar, The Story of
- Ahilyabai Holkar ... Ahmed, Fakhruddin Ali
- Ahmed, Humayun ... aid
- Aid to Displaced Persons ... Aïn Beïda
- Aïn Bessem ... Air New Zealand Limited
- Air Out Your Dirty Laundry ... Airedale Terrier
- Aires, Matias ... Aiyanār
- Aiyar, Mani Shankar ... Akaka Bill
- Akaka Falls ... Akhelóös Potamós
- Akhenaten ... Akron
- Akron Art Museum ... al-fil
- Al-Ḥamrāʾ Plateau ... ‘alalā
- Alalakh ... Alaska Pacific University
- Alaska Peninsula ... Albania, history of
- Albania, Orthodox Church of ... Albert I
- Albert I of Livonia ... Albery, Sir Bronson James
- Albery, Sir Donald Arthur Rolleston ... Albuquerque, Afonso de, the Great
- alburnum ... Alcibiades
- alcid ... aldehyde group
- aldehyde oxidase ... Aleksandrov, Pavel Sergeevich
- Aleksándrov, Pavel Sergeyevich ... Aleurocanthus woglumi
- Aleut ... Alexander, Francesca
- Alexander, Franz ... Alexis Falconieri, Saint
- Alexis I ... Alford guilty plea
- Alford plea ... ALGOL 60
- algology ... Alianza Patriótica para el Cambio
- Alianza Popular ... Alisjahbana, Takdir
- Alisma ... alkyl iodide
- alkylaluminum ... All Souls’ Day
- All Stalin’s Men ... Allagoptera
- Allah ... Allen, Bryan
- Allen, Byron ... Allgemeine Enzyklopädie der Wissenschaften und Künste
- Allgemeine Kulturgeschichte ... allitic crust
- Allium ... Alma, Battle of
- Alma-Ata ... Alnilam
- Alnus ... Alparslan
- alpen ... alpha-thalassemia
- alpha-tridymite ... Alsace
- Alsace-Champagne-Ardenne-Lorraine ... Alternanthera
- Alternanthera philoxeroides ... Alto Paraná
- Alto Uruguai River ... Alva
- Alva, Fernando Álvarez de Toledo y Pimentel, 3er duque de ... Ama-ga
- amabutho ... Amanita caesarea
- Amanita muscaria ... Amateur Hour
- amateur radio ... Ambala
- amban ... Amboy
- Amboyna ... Amen
- amen ... American alligator
- American Amateur Baseball Congress ... American Civil Liberties Union
- American civil rights movement ... American Foundation for AIDS Research
- American Foundation for the Blind ... American Journal of Psychiatry
- American Journal of Psychoanalysis ... American Pie
- American Pie 2 ... American Society for Horticultural Science
- American Society for Testing and Materials ... American, The
- American, The ... Amherst, William Pitt Amherst, lst Earl, Viscount Holmesdale, Baron Amherst Of Montreal
- Amhurst, Nicholas ... Amistad
- Amistad ... Amne Machin
- amnesia ... amorphous solid
- amorphous substance ... Amphisbaenia
- amphisbaenian ... Amsteg
- Amstel River ... amyl nitrite
- β-amylase ... Anaconda
- Anaconda ... analogue
- analogue computer ... Ananite
- Ananke ... Anastasius II
- Anastasius III ... ancestor worship
- Ancestor’s Tale, The ... Ancylidae
- Ancylopoda ... Andersen, Hjallis
- Andersen, Hjalmar ... Andersson, Bibi
- Andersson, Birgitta ... Andreae, Johann Valentin
- Andreaea ... Androclus
- androconium ... Aneides
- Aneirin ... Angel on the Roof, The
- Angel Pavement ... angiogenesis inhibitor
- angiography ... Anglo-American Cataloguing Rules
- Anglo-American Chain of Command in Western Europe, June 1944 ... Angra dos Reis
- Angra I ... anicca
- Anicetus, St. ... Animals Improvise Counterpoint, The
- Animals in That Country, The ... Ankylosauria
- Ankylosaurus ... Annapurna IV
- annates ... Annual Festival
- annual honesty ... Anomaluridae
- Anomaluromorpha ... Anredera cordifolia
- anṛta ... ant bear
- ant cow ... Antelope Valley California Poppy Reserve
- Antelope Wife, The ... Anthomyiidae
- Anthoniszoon van Riebeeck, Jan van ... Anti-Corn Law League
- anti-de Sitter/conformal field theory ... Anticleia
- anticlericalism ... antilock braking system
- antilogarithm ... antiphon
- Antiphon ... Antler orogeny
- antlerite ... Antropov, Aleksey
- antrum ... Anyang Art Park
- Anyathian complex ... AP
- AP ... apeiron
- Apeldoorn ... API gravity scale
- api, Le ... Apodanthes
- Apodeipnon ... Apologie for Poetrie, An
- Apologie oder Schutzschrift für die vernünftigen Verehrer Gottes ... apothecary
- apothecium ... Appendix Vergiliana
- Appennines ... applied microbiology
- Applied Minds ... Apted, Michael
- Aptenodytes ... aquamanile
- aquamarine ... Arab Bureaus
- Arab Deterrent Force ... Arabic numeral
- Arabic philosophy ... Aral Sea
- Aral Sea shovelnose sturgeon ... Araunah the Jebusite
- Arausio, Battle of ... arc furnace
- arc lamp ... Archaeosigillaria
- Archaeosporales ... archinephros
- Archinotic realm ... Arctic Monkeys
- Arctic National Wildlife Refuge ... Ardiente paciencia
- Ardiles, Osvaldo ... Arecibo River
- Arecomici ... argali
- Argall, Sir Samuel ... argon
- argon-40 ... Ariane et Barbe-Bleue
- Ariane, Princess ... ʿArīsh, Al-
- ʿArīsh, Convention of Al- ... Arizona State College at Flagstaff
- Arizona State University ... arm
- arm keel ... Armenian literature
- Armenian massacre of 1915 ... Armstrong Hot Seven
- Armstrong’s Last Goodnight ... Arnim, Harry, Graf von
- Arnim, Jürgen von ... Aronofsky, Darren
- Aronov, A. N. ... Arrest, Heinrich Louis d’
- arrestable offense ... Arsacid dynasty
- arsefoot ... art embroidery
- Art Ensemble of Chicago ... Artamene
- Artamène; ou, le grand Cyrus ... artesian flow
- artesian spring ... artificial aging
- artificial blood ... Artôt, Désirée
- Artôt, Marguerite-Joséphine-Désirée Montagney ... Arung Singkang
- Arunta ... AS-7 Kerry
- AS-8 ... Ascaphus
- Ascaphus truei ... ASE
- ASE ... Asheboro
- Ashendene Press ... ashrama
- ʿAshrāwī, Ḥanān ... Asian golden cat
- Asian gymnure ... ASL
- Asmar, Tall al- ... Aspergillus
- Aspergillus flavus ... Assassinations and assassination attempts involving U.S. presidents and presidential candidates
- Assassinations in the U.S. in the 21st Century ... assisted suicide
- assisted ventilation ... Assyrian Church
- Assyrian dialect ... Astonishing the Gods
- Astor family ... Astronomical Diary and Almanack
- Astronomical Journal, The ... At Fillmore East
- At Home Abroad ... Atashin, Faegheh
- Atasi, Faysal al- ... Athapaththu, Chamari
- Āthār aṣṣanādīd ... Athlone
- Athlone, Godard van Reede, 1st earl of ... Atlantic Coast Conference
- Atlantic Coastal Plain ... Atli
- Atli, Lay of ... Atomic Kid, The
- atomic layer epitaxy ... atrioventricular node
- atrioventricular valve ... Attenborough, Dickie
- Attenborough, Richard ... Atwater, Harvey LeRoy
- Atwater, Lee ... Audelay, John
- Auden, W.H. ... auger shell
- Auger yield ... Augustinus
- Augustinus Cornelii Jansenii, Episcopi, seu Doctrina Sancti Augustini de Humanae Naturae, Sanitate, Aegritudine, Medicina adversus Pelagianos et Massilienses ... Aurangabad
- Aurangzeb ... auscultation
- Ausdehnungslehre ... Australia, immigration to
- Australia–United Kingdom–United States ... Australian region
- Australian Republican Movement ... autecology
- auteur theory ... Autograph Man, The
- autogyro ... autonomous church
- autonomous sensory meridian response ... Ava
- Ava ... Aven, John Hamilton, Lord
- Avena ... Avey, Linda
- Aveyron ... Avon
- Avon ... awlbill
- awlīyāʾ ... axis
- Axis ... Áyioi Pándes monastery
- Áyion Óros ... Az ellenállás melankóliája
- aza ... azimuthally varying field cyclotron
- Azione, Squadre d’ ... Azzone Dei Porci
- Azzone Soldanus ... aʿyān
