Agentic: acting in the manner of, or having qualities of, an agent. Having the force of agency over a situation or person.
In this case, we are using 'agency' specifically to refer to power or control, 'agent' as an agent of change. An agentic leader takes an active leadership role, and is assertive, competitive, and independent.
In social cognition theory, the agentic perspective is that people live both as products and as agents (producers) of social systems. The agentic view stresses that exploring, manipulating, and influencing the environment is an important aspect of psychological and social development. (Ref, .pdf)
In contrast to these definitions, Stanley Milgram put forth the theory of 'agentic states' to explain why people would behave in inhumane ways in certain experiments. In an agentic state the person cedes their agency over to an authority, and does not hold agency themselves. "The essence of obedience consists in the fact that a person comes to view themselves as the instrument for carrying out another person's wishes, and they therefore no longer see themselves as responsible for their actions. Once this critical shift of viewpoint has occurred in the person, all of the essential features of obedience follow". (Ref).
At this point 'agentic' is still primarily used in the social sciences, particularly the fields of psychology and sociology.