Agonistic pluralism is the idea that we will never eliminate all the divisions and differences of opinion in society, and that it is unwise to try. It also means individuals can win and succeed, but not forever and not in everything. You can be President of the United States but not for more than eight years. You can build up a big corporation but not a monopoly.
Rather than try to destroy what are really irreducible differences of opinion; as liberalism, socialism, capitalism, and all the other political and economic ideologies attempt to do, agonism tries to find ways to accommodate disagreement and pluralism.
I stumbled across this concept today whilst reading Lady of Mazes by Karl Schroeder. This is a superb transhumanist, post-singularity, hard SF novel that deals with some interesting ideas about reality, democracy, and the future of humankind.
I strongly recommend everyone read this book. If you don't like it then I clearly do not have the same taste in literature as you do. Neither of us is right or wrong, so what does it matter?
Addendum: I know this doesn't affect irreducible differences, but there are some things that are considered (almost) universally bad and as such will still be considered bad in a agonistic situation.