Partisan means strongly supporting one side of an argument, political party, or group, often in a way that makes it hard to see the other side's point of view fairly. A partisan person is so loyal to their team, party, or position that they struggle to admit when the other side might have good points.

In politics, partisan behavior shows up when someone automatically supports everything their political party does while opposing everything the other party suggests, even before hearing the full idea. If two senators from different parties propose almost identical laws, a partisan voter might love one and hate the other simply because of which party proposed it.

You can see partisan thinking outside politics too. Imagine two students who always disagree just because they're on rival debate teams, or fans who insist their sports team never commits fouls while the opposing team fouls constantly. That's partisan thinking: letting loyalty override fairness.

The opposite of being partisan is being nonpartisan. Someone nonpartisan tries to judge each idea on its merits rather than which side proposed it. Bipartisan describes when people from two different sides work together, setting aside their partisan loyalties to solve a problem.

Partisan thinking can become a problem when it stops people from seeing the truth clearly or working with others to find good solutions.