a person actively engaged in an art, discipline, or profession, especially medicine.
If we google, it shows a practitioner is a person actively engaged in an art, discipline, or profession, especially medicine is a practitioner
In the same way, an Agile practitioner is a person who contributes to realizing Agile Values and principles using different practices like Scrum, XP, Kanban etc. A practitioner practices any of these practices based on the need of Agility for a given situation. He can devise any other practice if it best suited to realize Agile Values and principles for any specific environment.
It means he/she is actively engaged in the art of realizing Agile Values and principles, and how to implement is dependent on what is working well.