I see a 'midlife crisis' as a time when we realize that life is finite. It's often something that does indeed follow a 'crisis' in our lives. We might have faced redundancy, have seen a younger colleague promoted over us. We might be dealing with the emotions of an empty nest, a divorce or ending of a relationship.
Forty is a time contradiction and complexity, a.k.a. horrifying reality and inner turmoil. It's realizing that these are the days of questioning every decision, wondering about regrets, and realizing that life is short, and getting shorter every freaking day. It's when the terrifying reality sets in that yeah, THIS is life.
I believe that those of us who are called into a mid-life crisis and those who accept the honor of moving through it thoroughly and with grace are the lucky ones. Now that I am on the other side, I see my mid-life crisis as a true breakthrough, an awakening, and the path to the peaceful, joyful, and loving life I am living now.