I would define a cynic as someone who habitually assumes the worst of people. It’s pretty much the same as a pessimist, but a cynic is pessimistic about people rather than about reality in general. A cynic tends to assume that other people are lying or stupid. A skeptic knows that sometimes people are lying or stupid, but doesn’t leap to that conclusion.

Here’s an anecdote that illustrates the difference between a cynic and a skeptic. I once had a conversation with someone who reported that a friend of hers had been abducted by aliens – but when I asked if she believed the abduction really occured, she became very defensive and told me that her friend would never lie about something like that.

The notion that her friend might have lied was the last thing on my mind; I was thinking more along the lines of a bog-standard Susan-Blackmore-style hallucination. But had I thought “liar”, then I would have been a cynic, not a skeptic.