opacity controls the opacity =)
It is a range from 0 to 255. 0 being invisible and 255 being completely opaque.
To hide an object, a common practice is to set its opacity to 0. But these days, the 'visible' property is the better thing to use, as you don't have to track the previous opacity to restore it later.
To hide an object but still interact with it, it's common practice to set its opacity to 1. See also: object tracking
myObjectName.opacity = 200;