This is a Open in Popup 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 12,600 icons, all in the same style and quality.
This is an icon to indicate that a separate smaller window will open. There is a rectangle with rounded corners. In on of those corners there is an arrow with the head pointing to the corner directly diagonally.