A Clone is an individual artificially created to be identical to another person.
This happened to Josie Hunter. Josie was part of a trio of expeditionaries sent to assassinate Rudi, leader of the Advanced Cultivars. While Josie was away, the clone came and killed Ryan Docherty, and severely injuring Clark Johnson. This clone then went to Forthaven and appeared to be the real Josie. When Clark revealed that Josie attacked them, the clone was arrested. The real Josie returned after the clone escaped and took her three children. She was arrested but as she wasn't the clone, her children weren't with her. Cass Cromwell broke her out of jail to find her children, they managed to retrieve them and also encountered her copy who fled. (Episode 6)