What's the meaning of the "dynamic" in "high dynamic range"? What's the meaning of the "dynamic" in "high dynamic range"?

Why use this word?
Dynamic, adjective:
  
  1: pertaining to or characterized by energy or effective action; vigorously active or forceful; energetic:
  
  ...
  
  4: of or relating to the range of volume of musical sound.


HDR allows you to deal with high light energies. And such high energies could be characterized as being "vigorously active or forceful; energetic".

But I think the fourth definition is more suitable. Graphics and rendering are forms of signal processing, much like sound. So they share a lot in common. For example, the term "aliasing" originated in digital conversion of sound.

"High light energies" are the visual equivalent of "volume" in sound. So the "dynamic range" that you can record, the "range of volume" is high, as opposed to low.