To keep things simple here, I will refer to the term Middle Gray (or Grey) throughout this Blog. When I refer to this term I am not trying to be scientific and represent a certain percentage of reflected light.
I am merely referring to than greyscale tone that is exactly halfway between pure white and pure black. You might be able to refer to Middle Gray as Pure Gray.
Please do not over-analyze my use of halfway either. I am not trying to distinguish between a logrithmic value halfway between two tones, nor am I trying to distinguish between the relative luminance of a tone between zero light and the brightest light source.
Use the following image: take a strip of paper for each tone that can possibly be created between the colors black and white. This should be an infinite number of paper strips, but find the middle point and take that strip of paper. This tone is what I refer to as Middle Gray.