Let us note that the word "politics" refers to a category of human activity like "carpentry," "theater," or "surgery." What we really need to define is the term "political process." A political process is one in which you get a person or group of people to do what you want them to do. This is actually shorthand for a much more complicated set of ideas.
A political process is one in which two or more people adopt a single course of action. Typically, each individual involved in a political process maintains an agenda that is different from the group's collective agenda, or from any other individual's personal agenda. It is through the political process that an individual's agenda becomes publicized (if a free press is at work), negotiated and voted on (if a democracy), or implemented without further discussion (if the individual is the king). In every case, the individual engaged in a political process wants the group to adopt his or her agenda.
By the way, getting other people to do what you want them to do is also the job of leaders and managers. But you probably have already made that observation yourself.
Note that what you are trying to convince people to do can be viewed as either good or bad; that is irrelevant to the discussion. Of course, from your point of view, you are probably trying to get them to do the right thing, but this certainly depends on your perspective. Even when purely motivated, your objectives may be viewed as wrong, bad, misguided, or even evil by some other third party.