The term was first used during WWI, when the French press described Adolphe Pégoud as "L'as" (french word for Ace).

"Ace" is commonly used for pilots who shoot down a certain number of aircraft during combat. The precise number of aircraft varied in the beginning, but later (WWII) 5 victories became accepted as "standard".

If we take a closer look at the list of aces during WWII there is a huge difference between different airforces. For instance German WWII ace, Erich "Bubi" Hartmann , destroyed over 352 aircraft during the war, while the top American ace, Richard I. Bong , only has 40 kills.

This had multiple reasons:

• First of all the counting system used by various airforces. For instance, the German airforce credited a shared kill (two or more pilots shooting down one enemy aircraft, thus sharing the kill) to only one pilot. The French airforce fully credited shared kills to all pilots involved, while the American and British airforces equally divided their kills.
  The latter resulted in fractioned end results for a lot of aces (ex. Major George E. Preddy, Jr.'s final tally was 26.83). An aircraft shot down by 2 pilots counted for 0.5 kills, an aircraft shot down by 3 pilots counted for 0.33 kills, etc.
  
  
• The very nature of air combat also made verifying victories a problem. In most cases a victory would only count if a second party would confirm seeing the enemy aircraft crash, others simply took the pilot's word for it. The first method (confirmation by a wingman or other friendly aircraft) was too strict, in that quite a few of the enemy aircraft fell out of sight of other friendly forces, and it was not at all uncommon for a pilot and his wingman to be separated when the victory occurred.
  The second system held the potential for abuse if the pilots took advantage of it, and a few did.
  At a later stage, gun cameras finally eased this problem, but they didn't fully solve it. The cameras only ran when the guns were being fired, so if a target took a fatal hit but didn't burst into flame or have major pieces fall off, it might only be counted as damaged or a probable, not as a destroyed aircraft.
  
  
• There was a short period where American pilots were credited with kills for ground (strafing) victories, but this never really caught on. It was mainly thought of to encourage pilots in engaging enemy airfields and destroying aircraft on the ground. Towards true acedom, ground victories don't count.
  
  
• The main reason German pilots had such high kill rates is mainly because of the early years of dogfighting during WWII. In Russia and France they usually battled inferior fighters and ill trained fighter pilots. German pilots also flew more missions then axis pilots during the war: they usually fought untill they were eventually shot down whereas US pilots often rotaded back home after one or more succesful tours (to become instructors or simply for public relations reasons).

The Russian airforce is the only airforce which had female aces: Lydia Litvyak & Katya Budanova . They each scored 11 victories.

Ace in a day

The term "ace in a day" is used for pilots who shot down 5 or more enemy aircraft during a single day. The highest number of aerial victories for a single day was claimed by Emil Lang, who claimed 18 Soviet fighters on November 3, 1943. During WWII, 68 US pilots were credited with the feat.