If you had asked me when I was 30 what a great leader was, I could not have told you. I knew when I worked for someone that I thought was a great leader and I DEFINITELY knew when I worked for someone who I thought was a lousy leader, but I didn't have a definition that meant anything to me. It took me until I was in my 40s to figure it out, so here it is.
Great leaders create and sustain an ecosystem where every person is contributing at their highest possible level.
That's it. Every word in this has been tested and retested based on my (and others') experience and each one matters. But defining leadership is only the start. You have to be able to operationalize it. And when the definition calls out the concept of an ecosystem, what you are really saying is - this definition has to be shared by everyone in the organization. Because at the heart of this definition is the idea that the dynamic interaction, the behaviors, of every person in the org is what allows every person to contribute at their highest possible level - not just how management behaves. Figuring out how to operationalize it took another couple of years, but it starts with the idea of the Team.