I think in general, a decoy is an attempt to pull some piece away from an area of the board. I think it is normally an unforced line/continuation. The defending side can decide not to take or bother the decoy.
Overloading is creating a position where one side can win material since the overloaded piece can't successfully defend all threats.
Deflection is normally a forced line to get a defender to move away from a protected piece or square.
The word "decoy" is used in actual warfare when the enemy army does not have knowledge of the enemy's position. In chess, we have perfect knowledge of the enemy's position. This is not irreconcilable, but it can be confusing or unclear.
The example in the documentation is here:
https://www.chess.com/article/view/chess-tactics--definitions-and-examples#decoydeflection
Is the Black queen not simply overloaded? It has to keep the pawn pinned, and it has to go capture the enemy queen, so it's overloaded I think.
I don't see any essential difference between the Decoy example and the Overloading example:
https://www.chess.com/article/view/chess-tactics--definitions-and-examples#overloaded
Why can we not say the Bishop is being a "decoy" in that situation??
Reconciliation can be attempted by imagining the Black Q doesn't have perfect knowledge of the game situation, since she's just a chess piece, not the player. But in a chess AI the Black Q would be considered part of a larger consciousness that is focused on playing the game (e.g. Marvin Minsky's "Society of Mind" illustrates this somewhat.)
In conclusion, I think the whole category of "decoy" is a mistake.
There may be some arguments about "deflection" also being a duplicate of "overloading" but I am not so concerned about them.
Thank you for reading!
Cavatine