When something is unreliable, you can't count on it to work properly or show up when needed. An unreliable alarm clock might go off at the wrong time, or not at all, making you late for school. An unreliable friend might promise to help with your project but then forget or make excuses.

Think of reliability as a measure of how consistently something does what it's supposed to do. A reliable car starts every morning; an unreliable one leaves you stranded. A reliable source for a research paper gives accurate information; an unreliable source might contain mistakes or made-up facts.

People, machines, information, and even weather forecasts can all be unreliable. When your teacher says a website is unreliable, she means you shouldn't trust the information there without checking other sources. When someone calls a witness unreliable, they mean that person's memory or honesty can't be trusted.

Being unreliable is different from making one honest mistake. It's a pattern: something unreliable fails or disappoints again and again.