This Tick Box icon is a responsive icon. This means that it's optimized for four resolutions, from tiny 16x16 until highly detailed 80x80. The style comes from the latest Microsoft Office, which makes it perfect for any Windows applications, in particular, those with ribbon toolbar control. Unlike other icon packs that have merely hundreds of icons, this monster icon pack contains 15,000 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.