In one of his magazine articles on threats, Purdy wrote that assuming it's your move then a threat is a move your opponent could make if it were his turn to move that would be damaging to your game.
He pointed out a threat is a MOVE and to think of threats in words such as, "He threatens to win my Queen." or "He is threatening a mating attack." is not only vague, but is plain wrong! In order to determine if the threat is real or imaginary, you must calculate what would happen if your opponent actually made the move.
A good example of what Purdy was talking about can be seen in the famous game Yates vs. Reti, New York, 1924. You can play over the whole game HERE, but I want to look at the position from Reti's perspective after 11.Re1 and apply Purdy's advice. So, in the below position, it's actually black's move, but taking Purdy's advice, we will pretend it's white's move and see if Bxe6 is a real threat.