This Tick Box 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 5,300 icons, all in the same style and quality.
This is a checkmark inside of a square with rounded corners. The square has four normal sides and three normal corners, but the top-right corner is open. The checkmark's tail would meet the top right corner if the square was complete.