A fanatic is someone whose enthusiasm for something has become so extreme that it controls their thinking and behavior in unhealthy ways. While it's wonderful to care deeply about things that matter to you, a fanatic takes that devotion so far that they can't see other perspectives or think clearly anymore.

The word originally described people with extreme religious beliefs who couldn't tolerate anyone who disagreed with them. Today it applies more broadly. A sports fanatic might plan their entire life around their team's schedule and become genuinely upset when someone roots for a rival. A fanatic about healthy eating might refuse to attend a birthday party because the cake doesn't meet their strict rules, missing the celebration entirely.

Notice the difference between passion and fanaticism. A passionate chess player loves the game, studies it seriously, and plays whenever possible, but still has other interests and respects people who prefer checkers. A chess fanatic might insist that chess is the only worthwhile game and look down on anyone who doesn't share their obsession.

The related word fanatical describes this extreme quality: “She had a fanatical devotion to practicing piano, refusing to take even one day off.” When people become fanatical about something, they can lose their sense of balance and proportion. They can't laugh at themselves or admit when they might be wrong.