This definition overlaps with the definition of the quality known as toleration. Acceptance and tolerance are not synonyms.
Eckhart Tolle (The Power of Now, etc.) defines acceptance as a "this is it" response to anything occurring in any moment of life. There, strength, peace and serenity are available when one stops struggling to resist, or hang on tightly to what is so in any given moment. What do I have right now? Now what am I experiencing? The point is, can one be sad when one is sad, afraid when afraid, silly when silly, happy when happy, judgmental when judgmental, overthinking when overthinking, serene when serene, etc.