Idea of Opacity Icon
The image is a square that is made up of a lot of little squares. Some of the squares are darkened and other are not. Along the outer edge of the bigger square, some of the squares are opened on one side.
This Opacity icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,100 icons, all in the same style and quality.