Opacity
The degree to which a layer or adjustment obscures what is beneath it, expressed as a percentage.
Measured in
%
In depth
Opacity is the simplest and most underused refinement in editing: almost every adjustment benefits from being made too strong and then reduced, because judging a subtle change against nothing is much harder than reducing an obvious one. It also lets several strong effects coexist without any one dominating.
Examples in practice
A sharpening layer applied at full strength then dropped to 40 per cent gives a result that is easier to judge than sharpening cautiously.