Paranoia is an irrational fear that others are trying to harm you, even when there's no real evidence for it. Someone experiencing paranoia might believe their classmates are plotting against them when they're just having a normal conversation, or think a teacher is purposely trying to make them fail when the teacher is simply doing their job.

Paranoia isn't the same as being cautious or careful. If you check that your bike lock is secure because bikes get stolen in your neighborhood, that's reasonable. But if you think someone is specifically following you every day to steal your bike, studying your routine and planning an elaborate theft, that's paranoia.

In severe cases, paranoia can become a serious mental health condition requiring treatment. But people also use the word more casually: “I'm being paranoid, but I keep thinking I forgot to turn in my homework” means you're worried about something that probably didn't happen.

The adjective form is paranoid. Someone who frequently experiences paranoia might be described as having a paranoid personality. Paranoia makes life exhausting because you're constantly suspicious and on guard against threats that don't exist. It can damage relationships because it's hard to trust or connect with others when you believe they're against you.