Technical term used in physics, imaging, and design to describe how opaque a material or layer is. Often contrasted with 透明度 (transparency).
In this image editing software, you can adjust the opacity of the layer.
The higher the opacity, the less the background shows through.
透明度 means transparency, the opposite of opacity. 不透明度 is the degree of opaqueness, while 透明度 is the degree of clarity or see-through quality.
Compound of 不 (non-) + 透明 (transparent) + 度 (degree). The word is a modern technical coinage, likely from the early 20th century, following the pattern of similar scientific terms.