Term | Explanation |
| A-Force | British intelligence organization responsible for deception and espionage in the Mediterranean and Middle East. |
| A4 | Aggregat 4 - earlier name for the German V2 rocket. |
| Abwehr | Intelligence service for the German Armed Forces run by Admiral Canaris. |
| Accolade | Operational code-name for Allied attempt to capture the Dodecanese Islands in the Agean. |
| Acrobat | Code name for British advance on Tripoli in 1941. |
| Adlertag | Code name for the day of intense German air attack on Britain (Eagle Day) 15th September 1940. |
| AEF | Allied Expeditionary Force. |
| AK | Armja Krajowa (Home Army) – The Polish Resistance. |
| Aktion T4 | Extermination of mentally ill and handicapped patients by the Nazi authorities. (Named after Tiergartenstrasse 4, the address of Nazi Central Office in Berlin.) |
| Allgemeine SS | Standard branch of the SS, as opposed to the military or armed Waffen SS. |
| Allies | Those countries fighting against the Axis powers. i.e. Britain, France, USA, Canada, USSR etc. |
| Altreich | Term used to describe Germany before the annexation of Austria. |
| Amt Mil | German Army intelligence organization which succeeded the Abwehr. |
| Anakim | Code name for the Allied re-occupation of Burma in 1944. |
| Anschluss | German annexation of Austria in March 1938. |
| Anthropoid | Operational code name for the assassination of Heydrich in Prague. |
| Anti-Comintern Pact | Agreement by Germany, Japan and Italy to oppose the Communist International organisation. |
| Anton | Code name for the German occupation of Vichy France, November 1942 - later known as Atilla. |
| Anvil | Operational code name for the Allied invasion of the South of France, August 1944. |
| ANZAC | Australia and New Zealand Army Corps. |
| Arcadia | Code name for the first Anglo-American conference in Washington, December 1941. |
| Argonaut | Code name for the Anglo-American-Soviet conference at Yalta, February 1945. |
| Argument | Operational code name for Allied air attacks on German factories in February 1944. |
| Ariel | Operational code name for the evacuation of France by the Allied troops. |
| Aspirin | British equipment used for jamming German Knickebein radar |
| Atlantic Charter | Anglo-American declaration that stated the countries aims for the outcome of the war. |
| Aufbau Ost | Operational code name for the German build-up of arms prior to the invasion of the Soviet Union. |
| August Storm | Operational code name for the Soviet invasion of Manchuria, August 1945. |
| Avalanche | Operational code name for the Allied invasion at Salerno, September 1943. |
| Avonmouth | Operational code name for the Allied attacks in Narvik, May 1940. |
| Axis | Alliance between Germany, Italy and Japan. Originally the Rome-Berlin Axis (i.e. a North-South line between the two capitals), later became the Rome-Berlin-Tokyo Axis. |