This is a Checked icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,500 icons, all in the same style and quality.
The icon is shaped like a circle but the top right corner of it doesn't fully connect. At the center of the circle is a check mark. The end of the check mark sticks out the opening in the circle a bit.