A skeptic is a person who doesn't believe something is true unless they see evidence. As a skeptic, you refuse to believe your sister saw a ghost — after all, she can't prove it.
Skeptics are doubters — they need to see proof before they will believe. If you're a skeptic, you're probably dubious about things like astrology and magic. Some skeptics feel the same way about religion, preferring facts that can be scientifically and historically proven. Coming from the Greek word skeptikos, which means "thoughtful or inquiring," it's no surprise that a skeptic is someone who asks a lot of questions — and isn't easily convinced.