HDR = High Dynamic Range
I am not that smart on the subject so I cheated. I looked it up...
› What is Dynamic Range? The dynamic range is the ratio between the maximum and minimum values of a physical measurement. Its definition depends on what the dynamic range refers to.
For a scene: ratio between the brightest and darkest parts of the scene.
For a camera: ratio of saturation to noise. More specifically, ratio of the intensity that just saturates the camera to the intensity that just lifts the camera response one standard deviation above camera noise.
For a display: ratio between the maximum and minimum intensities emitted from the screen.