The faster the action the more the rod initially flexes towards the tip. Most usually you see fast meaning that it flexes initially in the upper 1/3rd, medium is in the upper 1/2 and slow is one that even initially flexes all the way down to the handle So some fishermen combine terms when the action is on the borderline between any two of these.

I'd also agree that the AA degree ratings of the Common Cents System make a hell of a lot more sense as you can see where the rod flexes and put an actual number on it. Really helps when making comparisions.

And remember that all blanks will flex into the butt area if you put enough load on them. Action is just where the blank initially bends.