Here's the fourth and final criterion for making the Fortune Most Powerful Women List: social and cultural influence. It's certainly more difficult to measure than revenues or profits, the health of a business, or even the arc of a woman leader's career. But vast social and cultural influence is precisely why Oprah Winfrey ranks No. 8 on our list. Her company, Harpo, is privately held and nowhere near the size of what other MPWomen control (typically businesses with at least $6 billion in sales). But given Oprah's media assets -- O, The Oprah Magazine, her own TV show, and her interests in Dr. Phil, Rachael Ray, and more -- you see why we say she's more powerful even than some Fortune 500 CEOs, such as Sara Lee's (SLE) Brenda Barnes.

P.S. Another media mogul who used to be on the MPWomen list but is no more: Martha Stewart (MSO). She fell off in 2002 when she tumbled into her legal turmoil.

Click here to see video excerpts of a Most Powerful Women panel on leadership that we did in New York in May. The panel includes Ursula Burns, president of Xerox; Andrea Jung, CEO of Avon; and Ann Moore, CEO of Time Inc.