Click injection is a type of ad fraud where fraudsters misattribute a genuine install to the fraudulent click by replacing a legitimate click with a fraudulent one. It happens just after an install and the aim here is taking credit for installs. It is an attribution fraud presented on Android devices in which a fraudulent application or a method sends a fraudulent click to imitate an install. 

Measuring CTIT (Click to Install Time) is an effective way to detect click injection. If CTIT is very short, it might be an indicator for click injection. While MMPs(aka attribution tools) are trying to detect and prevent click injection, there are also third party mobile ad fraud tools analyzing impressions, clicks, installs, and events to detect and prevent mobile ad fraud in real time.