Leadership - What is it exactly?

This afternoon, I was talking about leadership within the context of my Business Project Management class at CIS-Endicott. While introducing the subject to my MBA students, I reflected on how leadership is such a complicated subject. There is no agreement on a single definition of leadership, even though is probably one the most covered issues in fields such as psychology, sociology, business management, economics and history, among others. A very broad definition points out at leadership as the "ability to influence a group toward the achievement of goals". What do you think? What exactly is leadership and how will you define it?

I remembered reading an interesting article about the subject in HBS Working Knowledge a while ago. Here is the link: http://hbswk.hbs.edu/item/6372.html