Delusional means believing something that isn't true, even when clear evidence shows otherwise. A delusional person holds onto false beliefs so strongly that facts and reason can't change their mind.

In everyday conversation, people sometimes use this word loosely. If your friend insists they're the best basketball player in school despite losing every game, someone might jokingly say, “You're delusional!” But true delusions are more serious: they involve beliefs that are genuinely impossible or contradicted by reality, and the person can't recognize that they're wrong.

Doctors use this term to describe a symptom of certain mental illnesses, where someone might believe things like they have magical powers or that people are plotting against them when neither is true. These are fixed false beliefs that the person defends despite overwhelming evidence to the contrary.

When someone is delusional, it's as if their own mind is deceiving them. This helps explain why you can't simply argue someone out of a delusion: they're not being stubborn, their perception of reality has become distorted.