According to The Scrum Guide™, “The Scrum Master is a servant-leader for the Scrum Team”. There are many articles on the servant meaning in the Scrum Master role. In theory leader does not have to be servant. Traditional leader is not servant . The purpose here is to take a short look on these leader-related terms and realize how they are tend to be understood on a daily basis. Today the meaning of a leader goes towards the servant role. There is a common understanding of being to serve others rather than giving commands. It is not addressed only to Scrum Masters, this role is only an example.
Let’s have a look on some publications that show todays meanings of a leader and a boss.
In the 5 Leadership Lessons I Learned Working For Mayor Bloomberg article there is a statement “He cared about the people he would serve” . The 7 Characteristics That Separate A Boss From A Leader text says: “While a leader can be a boss, not every boss is a leader.”  and Why You Should Stop Being a Boss and Start Being a Leader says: “boss thinks in terms of him or herself; a leader thinks in terms of we. “ .
A boss is seen as a traditional leader who manages and does not participate in work. A servant-leader is a person who cares for other people and work together with people in order to achieve a goal. The servant word is in many cases an assumption when considering a leader role. We expect a leader to be servant and therefore we would like to have a servant-leader every time we call someone a leader. Scrum Master is for sure a servant-leader. The Scrum Guide™  describes this role with service expectations.