It's an icon representing a medical thermometer. The thermometer is pointing down and to the left. There are hash marks on it to show where the temperature would be and a line going from the base through the center of the thermometer.
This is a Warm 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 13,900 icons, all in the same style and quality.