What is a Queen stage in cycling?

A "Queen" stage is the hardest climbing stage of a multi-day stage race. Usually it's not explicitly stated as the Queen stage, and more implicitly known. There is sometimes debate between which two stages should claim the crown of the hardest climbing stage.

One could calculate it based on elevation gain but that doesn't necessarily take into account hard climbs. An extremely rouleur based stage could have a bunch of elevation gain (by going up and down) but no extremely hard climbs anywhere throughout the stage.

Queen stages usually offer up a defining moment for the general classification time battle between the main contenders given that the most time can be won or lost or the hardest mountain stages.