Fanaticism is extreme, unreasonable devotion to a belief, cause, or activity that makes someone unable to see other viewpoints or act with common sense. A fanatic is so intensely focused on their passion that they lose perspective and may become rigid, intolerant, or even hostile toward anyone who disagrees.

You can see the difference between healthy enthusiasm and fanaticism in how people react to disagreement. Someone who loves basketball might talk about it constantly and never miss a game, but they can still be friends with people who prefer other sports. A fanatic about basketball might refuse to be friends with anyone who doesn't share their passion, or insist that basketball is the only sport worth playing and that all others are worthless.

Fanaticism often involves closing yourself off from new information or different perspectives. A student fanatic about a particular video game might refuse to try any others, insisting theirs is perfect despite never having experienced alternatives. Political or religious fanatics can become so certain they're right that they can't imagine other people might have valid reasons for thinking differently.