This is a Markdown 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 of Markdown is shown as a rectangle. The rectangle has slightly rounded edges. In the center of the rectangle on the left side is a large capital letter M, and next to that is a downward facing arrow.