What is the correct definition of a set in fitness?
one group of repetitions
an hour of exercise
a muscle contraction
ability to exert force

I would say that the answer is one group of repetitions

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)