Detection, Recognition, and Identification (DRI) are guidelines that have been set in place by The Night Vision Thermal Imaging Systems Performance Model, also referred to as the Johnson criteria. They are a universally accepted set of standards developed by the US Army to provide a means of measuring the distance whereby a thermal sensor can produce an image of a specific target.
Detection means that you will be able to see the target, however it will be little more than a speck. Specifically it means that the target is visible on at least two pixels, and that there is a good chance that the target is actually something of suspicion.
For a complete explanation of DRI measurements and what to expect from them, please see our Whitepaper on DRI: Thermal Distance Ratings Explained.