Leadership is that process in which one person sets the purpose or direction for one or more other persons, and gets them to move along together with him or her and with each other in that direction with competence and full commitment.
- Elliott Jaques and Stephen D. Clement in Executive Leadership: A Practical Guide to Managing Complexity
My biggest reservation is with the "with competence and full commitment" part.
Leadership, like management, is an assortment of responsibilities that may or may not be executed well. There are great leaders, terrible leaders, and a huge number in-between.