A leader is one or more people who selects, equips, trains, and influences one or more follower(s) who have diverse gifts, abilities, and skills and focuses the follower(s) to the organization’s mission and objectives causing the follower(s) to willingly and enthusiastically expend spiritual, emotional, and physical energy in a concerted coordinated effort to achieve the organizational mission and objectives.
Winston, B. E. and K. Patterson. 2006. "An Integrative Definition of Leadership." International Journal of Leadership Studies,: 1(2):6-66.
Many definitions of leader exist such a the simple Merriam-Webster definition of "a person who leads as a guide, conductor." However, the definition provided above gives a more complete and elucidative definition that is more useful in the context of leaders of teams in organizations.