Some Say There Are Almost as Many Definitions of Leadership

Some say there are almost as many definitions of leadership as there are people who have tried to define the concept. Although different definitions sound similar, they have important differences. Compare and contrast these definitions, for example:
Alistair: A leader is someone who can influence others and who has managerial authority.
Bentham: Leaders have the ability to influence a group toward the achievement of a vision or set of goals.
Cooke: Leaders exercise influence and control over other members to help a group or organization achieve the organization's goals.
Which of the following would be a leader according to Bentham's definition of leadership but would NOT be a leader according to Alistair's definition of leadership?
A CEO who is able to shape the direction of a large organization
An executive who is unable to affect the actions of others due to fundamental concerns about his competence
A blogger with a wide audience who is effective at convincing others to adopt her political opinions
An isolated individual with strong opinions about the most effective ways to run a business while respecting codes of ethics
A middle manager who directs those under her authority to achieve the goals defined by top management