Thermal: ther·mal; ˈTHərməl/
1. of or relating to heat.
Imaging: im·age; ˈimij/
verb: gerund or present participle
1. make a representation of the external form of.
We make images of heat. Think about how an image is made with your smartphone camera. Visible light reflects off the objects you want to image, that light comes into a lens, it is focused on a sensor, and an image is delivered. We do the same thing with heat coming from the object. We have been thermal imaging for close to 25 years, since the first solid-state thermal imagers were introduced. You might wonder how do you focus heat? What kind of sensor detects heat? How do you display heat? All good questions.
Just as in any science or technology, there are armies of brilliant people making thermal imaging products better all the time. Just like the evolution from early PCs with C:\> to how we function with smartphones, thermal imagers have come a long way. But very simply, thermal imaging is your thermal scene, a lens, a sensor, and a display, simple as that.