A culprit is a person who does something wrong, like committing a crime. When your wallet got stolen out of your pocket, there was a culprit to blame in the crowd.
The word culprit comes from the Anglo French words cul prit, a contraction of culpable, which means "deserving blame." A culprit isn't always a person; it can be anything that caused something bad to happen — even a sock left on the steps can be a culprit if it makes you slip and fall. Don't confuse culprit with scapegoat, which is a person or thing that gets blamed for something it didn't do.