This is a Stream 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 12,900 icons, all in the same style and quality.
The icon is typically seen in loading animations for websites. It's a circle made up of smaller circles around the outside. One circle is larger, and they gradually get smaller as they progress around the outer circle.