What does the word "refractory" in "refractory period" refer to?
From encyclopedia Britannica:


  "After repolarization there is a period during which a second action
  potential cannot be initiated, no matter how large a stimulus current
  is applied to the neuron. This is called the absolute refractory
  period, and it is followed by a relative refractory period, during
  which another action potential can be generated, but only by a greater
  stimulus current than that originally needed. This period is followed
  by the return of the neuronal properties to the threshold levels
  originally required for the initiation of action potentials."


Absolute refractory period = initiation impossible absolutely

Relative refractory period = initiation possible but in greater energy relative to the original required.