A leader is a person who leads a given project. Anybody can create a project and becomes the self-assigned leader of this project. A project needs a leader to become alive. A project becomes orphaned when its leader dies without having designated a successor.
The leader of a project can work alone on his project, or decide to form a team, i.e. to delegate parts of his work to other persons, called workers. He does this by defining a number of positions and by engaging a worker for each of these positions.
For the outside world, the leader remains the main and only responsible. When something goes wrong, the leader is to blame. The leader decides how he distributes critics and praise, shame and fame, profit and loss among his workers.
The act of forming a team is not reserved to the leader. Every worker of a team can himself form a team for doing his work. A position can be considered a sub-project or a “child” of its “parent” project. The fundamental difference between a top-level project and a sub-project is that its leader is not self-assigned and can be replaced by somebody else by the leader of the parent project.
A big question is how to organize the succession of the leader of a given top-level project. Ideally the leader designates his successor while he is alive. But this is not always feasible. For example if the leader dies unexpectedly without leaving any inheritor.
Some projects need by definition multiple leaders.
For example, a natural family is a project with two de-facto leaders, the mother and the father. “Patriarchal” law systems consider the father as the one and only leader. But even in “modern western” law systems, where both parents are considered as peers with equal rights and responsibilities, administration can require a single contact person called the head of household.
A modern nation state should implement separation of powers, that is, have three branches: a legislature, an executive, and a judiciary. Each of these branches should have its own leader. On the other hand, most modern nation states try to find a single leader, called the president. Which leads to division or even dictatorship.