Definition
The defiant power of the spirit (Ger. Trotzmacht des Geistes) is the human capacity to say “no” to one's drives, fears and moods and “yes” to values - in spite of everything.
The opposite of fatalism
Frankl contrasted it with the attitude “that's just how I am, I can't help it”. A person has a character but is not their character - they can take a stand towards it and change themselves.
Saying yes to life in spite of everything
The title of one of Frankl's books - …trotzdem Ja zum Leben sagen (“Saying Yes to Life in Spite of Everything”) - is the briefest expression of this idea. It comes from a song sung by prisoners of Buchenwald.
In therapy
The logotherapist appeals to this power, especially with anxiety, obsessions or addiction: the symptom does not have the last word.
Sources
V. E. Frankl, The Doctor and the Soul; Homo patiens.