A
- Achilles, Greek mythological character — Achilles' heel, Achilles tendon
- Adam, Biblical character — Adam's apple
- Adam Walsh, Abduction-Murder Victim — Code Adam
- Alvin Adams (1804–1877) — Adams Express
- Thomas Addison — Addison's disease
- Queen Adelaide of Saxe-Meiningen (wife of King William IV) - the city of Adelaide in Australia
- Len Adleman — the third letter of the name RSA, an asymmetric algorithm for public key cryptography, is taken from Adleman
- Agrippina the Younger — Cologne, Germany (formerly Colonia Agrippina)
- Alfred V. Aho — the first letter of the name awk, a computer pattern/action language, is taken from Aho
- Semyon Alapin — Alapin's Opening
- Adolf Albin — Albin Countergambit
- Alexander Alekhine — Alekhine's Defence
- Matthew Algie — tea and coffee merchant company
- Alice Liddell — Alice in Wonderland, Alice in Wonderland syndrome
- Alice Roosevelt — Alice blue, said to be the color of her eyes
- Alois Alzheimer — Alzheimer's disease
- Albert, Prince Consort — Prince Albert piercing, a common form of male genital piercing; Alberta (Canada)
- Herb Alpert and Jerry Moss — A&M Records
- Arthur Cecil Alport — Alport syndrome
- Bruce Ames — Ames Test, which tests for carcinogens
- André-Marie Ampère — ampere — unit of electric current, Ampère's law
- Roald Amundsen — Amundsen Sea; Amundsen crater, a crater on the Moon; Amundsen-Scott South Pole Station
- José de Anchieta — Anchieta Island, Anchieta Highway, in Brazil
- Anders Jonas Ångström — angstrom, unit of distance
- Adolf Anderssen — Anderssen's Opening
- Virginia Apgar — the Apgar score, used to determine the general health of neonates
- Antoninus Pius - Antonine Wall
- Saint Thomas Aquinas — many educational institutions
- Rafael Moreno Aranzadi, nicknamed Pichichi — The Pichichi Trophy
- Archimedes — Archimedes' screw, Archimedes' principle, Archimedean point
- William George Armstrong — Armstrong breech-loading gun
- Hans Asperger — Asperger syndrome
- Robert Atkins (nutritionist) — Atkins Diet
- Atlas, a Titan who carried the sky on his shoulders — atlas
- Aurélio Buarque de Holanda — Aurélio's Brazilian Portuguese Dictionary.
- Augustus Caesar — the month of August; the city of Zaragoza (originally Caesaraugustus); the city of Caesarea in Israel; numerous other cities once named Caesarea; the Caesarean section, because he was supposedly born in this manner
- R. Stanton Avery — Avery Dennison Corporation
- Amedeo Avogadro — Avogadro's number, Avogadro's Law
Read more about this topic: List Of Eponyms (A-K)