This is a Ctrl 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 icon for control is a rectangle with sides that are nearly equal in length. The width is a bit larger than the height. The corners are rounded, and at its center are there letters ctrl, with the C being capitalized.