What does power really mean to women?

Looking back on 15 Most Powerful Women lists and the shifting definition of "power."

FORTUNE -- Here is what I learned from being present at the creation of Fortune Most Powerful Women in 1998 and helping to produce the annual MPW list 15 times.

Power is what you make it.

And Power, in the minds of the Fortune MPW, has changed greatly.

Let me explain, by taking you back to MPW's beginnings. MPW started, actually, with a trip to New Jersey in the summer of 1998. I visited Lucent Technologies, then a red-hot telecom giant, to interview the two most senior women there. One was a well-known executive who had turned around businesses inside AT&T (T): Lucent EVP Pat Russo. The other was a woman few people outside of telecom had heard of. Her name was Carly Fiorina.

When the 44-year-old group president of Lucent's Global Services Provider business told me her story that day, I was beyond impressed. Fiorina had dropped out of law school, started as a secretary, and risen to head Lucent's largest division. By Fortune's criteria -- the size and importance of the woman's business in the global economy, the health and direction of the business, the arc of the women's career, social and cultural influence -- Fiorina possessed more power than Oprah Winfrey. We named Oprah No. 2 that first year. We made Carly No. 1 and put her on Fortune's cover.

When she scored the CEO job at Hewlett-Packard (HPQ) the following summer, it was a stunning advance for women, but Fiorina felt anxiety about her power. "My strength is my strength, but it also can be a weakness," she later told me, as she struggled to hold on at HP. Her leadership style came across as too aggressive to many. In 2005, the board fired her.

The band of acceptable behavior for women leaders was, back then, even narrower than it is today. No question, aggressive women are judged more harshly than men tend to be. To deal with that reality, many women succeed by deploying a gentler brand of power.

Meg Whitman, as CEO of eBay (EBAY) (Fiorina's successor at No. 1 on the MPW list), was nicknamed "mom" by her senior team. Later, running for governor of California and taking charge at troubled HP, she necessarily toughened.

Anne Mulcahy, who saved Xerox (XRX) from bankruptcy, used to define power as "influence" -- "so it doesn't feel like power. It feels like consensus," she said. It took a years of being in charge, but "I've learned that a decision needs to be made. A call needs to be made...I'm still learning."

And then there is Oprah. When I interviewed her for a Fortune cover story, Oprah Inc., in 2002, she disliked the word "power" and refused to call herself a businesswoman. ("If I'm a businesswoman and a brand, where is my authentic self?" she asked.) Eight years later, when I returned to Chicago to talk with her about launching her cable TV network, OWN, she told me, "I accept that I'm a brand" -- and owned her "power."

Own your power. That's what I told Facebook (FB) COO Sheryl Sandberg the first time I met her, when she was the top woman at Google. Ken Auletta captured the moment in his 2011 profile of Sandberg in The New Yorker:

"Sandberg says that she had an "Aha!" moment in 2005, when Pattie Sellers, an editor at large at Fortune, invited her to the magazine's Most Powerful Women Summit, an annual gathering of several hundred women. Sandberg attended, but she thought the title was embarrassing, and refused to list it on the Web-based calendar that she shared with her colleagues. She says that Sellers later chided her for being timid [and asked] 'What's wrong with owning your power?'"

Sandberg urged young women to own their power in her 2009 essay for Fortune: "Don't Leave Before You Leave." Today, she is the world's most visible cheerleader for aspiring women, challenging them to take risks and "lean in" to their careers.

This year, the spotlight shines on two MPW who also have learned to own their power. IBM (IBM) CEO Ginni Rometty, Fortune's new No. 1 on the list, admitted at last year's Most Powerful Women Summit that early in her career when a boss offered her a promotion, she told him that she didn't feel ready. Actually, her husband gave her the "aha moment": "Do you think a man would have ever answered that question that way?" he asked her. Rometty accepted the promotion.

And vaulting up this year's Fortune MPW rankings, to No. 14: Marissa Mayer. In July, after accepting the CEO job at Yahoo (YHOO), the former Google (GOOG) executive revealed to Fortune: "I'm pregnant." A self-described shy "nerd" from Wausau, Wisconsin, Mayer is not only the youngest woman ever to make the MPW list. At 37, she is also the youngest CEO of a Fortune 500 company and the first to assume that top job pregnant.

The fact that the Yahoo board -- and the world, which reacted mostly positively --welcomes a female chief with a complicated life is progress.

Yes, Power is evolving. Mayer's story evokes the definition of power that I've come to embrace: Real power is personal power. It's what you do and what you have beyond your job description and your tenure. Real power is what you do with your full life.