Acceptance, to me, is the absence of delusion--recognizing things for what they really are, instead of how I would like them to be, or how I am afraid they are.
Acceptance is the single most powerful tool I have in my life's toolbox today. It served me when I was dealing with an alcoholic relationship, and it serves me in my own sobriety. Once I accept something, I can take actions that have some actual effect on my life. Maybe not actions that will change the person or situation I was resisting, but actions that will make MY life better, regardless of the person or situation I can't change.