When does a card effect that reads 'once per turn' mean it can be either during your turn or your opponent's turn in Yu-Gi-Oh?

Answer 1

It depends on what the effect is.

If it is a Quick Effect, or a Triggered Effect, then it can be used once in either turn.

If it is an Ignition Effect (which it will be if it doesn't specifically respond to something, or specifically say it can be used in the opponent's turn) then it actually means 'Once per your turn'. While confusing, it'd technically be redundant to say 'once per your turn' on an Ignition Effect, they can only be activated in your own turn anyway.

Answer 2

It depends on the effect itself. A monster effect can only be activated in the opponent's turn if it specifically says so, or somehow implies that it is used in the opponent's turn. Battle Fader for example, is used when the opponent declares a direct attack, meaning it can only be used in the opponent's turn. It is therefore also a Quick Effect, spell speed 2.

Most monster effects though are Ignition Effects, which are spell speed 1 and can only be activated in your own turns. So if one of these had 'Once per turn' on it, then it is in fact once per your turns, the text is not allowing it to be used during the opponent's turn.

However, if the monster's effect does specifically say it can be used in the opponent's turn, and is therefore a Quick Effect, since Quick Effects can be used in both turns, a Quick 'once per turn' effect can be once in every turn.

Answer 3

It means only your turn