It is a light bulb. Inside of it, there is a “Y” sort of drawing to indicate the inner workings of the bulb. It also has the screwing part on the bottom of the bulb.
This is a Idea 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 13,300 icons, all in the same style and quality.