What was the requirement to be named an aviator ace in World War 2?

The standard has always been a pilot has to shoot at least five planes down(since ww-1). To become an "ace", you had to achieve five kills in air to air combat. And these had to be verified or witnessed kills. If you went off on a free chase and downed a "boggie" but there were no friendlies around to confirm it, you might not get credit for it. That is why sometimes in bios of fighter pilots you will see X number confirmed kills, Y number probable kills