What is Aristotle’s definition of politics? And do you agree with his definition?

As with all Greek thinkers of the period, Aristotle categorized human activity within three domains.

A human, on the most basic level, functions as an individual. The study of ethics (set out in Nicomachean Ethics) and of the nature of the individual soul (set out in De Anima) both pertain to the person as an individual, making his or her own moral choices.

Next, the human is part of a household or "oikos", which in antiquity would consist of husband, wife, unmarried children, slaves, and perhaps other unmarried female relatives. Our term, economics, derives from oeconomia, the study of the organization of the household, including its finances and the roles of different members of the household. 

Finally, in Greece, people lived in city states, or "poleis" (singlular: "polis"), which normally consisted of a densely occupied urban area, including residences, religious buildings, markets, administrative areas, and education, sporting, and cultural venues, surrounded by the "chora" or countryside, which consisted of farms, mines, forests, quarries, and in general any part of the polis which was not the central urban area. Politics, for Aristotle, was the study of the polis, or how humans interacted, organized themselves, governed, and made ethical choices within the context of groups larger in size than the oikos, which in ancient Greece would have meant the polis. 

For Aristotle, the central aim of the polis was the human good, and the study of politics (by which he meant what we know would term political philosophy) was practical, making decisions about what forms of government and legislation would most promote the human good.

There are two areas in which you can agree or disagree with Aristotle's definition. First you can agree or disagree with his notion of the goal of the state. Second, you can agree or disagree with his notion that the study of politics is practical and moral. Personally, I neither agree or disagree, believing that one can approach the study of politics in many different ways, and that different definitions and different modes of analysis are appropriate to different specific problems.