Version 3.5.8
Changing the Opacity
The opacity control can be found on the right side of the Effects Panel.
It is the first slider shown here:
What is Opacity?
Opacity controls how much you can "see through" the media.
NOTE: If you want to turn off the media altogether, don't set Opacity to zero, as this still uses CPU. You should turn off the visibility of the shot element.
Isn't that Transparency?
Strictly speaking transparency means to completely see through an object. In this case, we can partially see through the object, so it's called Opacity (or how Opaque the object is).