Is leadership defined by seniority?

No, leadership is not defined by seniority. 

Leadership is the process of guidance, influencing the behaviour of others in order to achieve a goal. Leaders develop their vision and further motivate subordinates to achieve that. It binds a group together and motivate them to achieve their goals. In leadership, a group of two or more people interact with each other and the group's behaviour is being moulded and shaped by an intelligent, mature leader only.

There are many people who talk about a company's leadership referring to the senior executives. Your understanding,intelligence and experience makes you a good leader, and not how senior you are. It is said that people who are senior are more experienced than the young ones. But, there are many who are fighting and are dedicated towards their work and see themselves as a good leader in the near future.  Leadership is not that you have when you will reach a specific pay grade, it can happen anytime.

And, leadership has nothing to do with titles. Your title does not define if you can be a leader or not. One can be a leader anywhere, whether it is their family, workplace etc.

One of the definitions of Leadership states that it is a process of social influence which maximizes the efforts of others, towards the achievement of a common goal.

There is no rule-book in which there is written that one needs to be at some specific age, position to lead others as it has got nothing to do with seniority. One needs to be smart, creative, compassionate to be a good leader no matter what their position or title is. Anyone, who possess these qualities, can become a leader. For instance, in any sports team Captaincy is given to the player who performs well not to the player with the most experience or senior player. In this, leadership skills also play important part in deciding a leader.

All this discussion comes to a conclusion that Leadership is not defined by seniority only.