An athame is a double-edged steel ceremonial knife that witches use to direct energy.
Serena Fredrick had an athame on her altar the night she was killed by Jeremy Burns. He killed her with his own jewel-encrusted athame. After her murder, Andy Trudeau asks if Serena was killed with an athame and his partner describes the murder weapon as a knife with two edges. Andy told him that that was an athame. Jeremy also attacks Piper Halliwell with his athame in an elevator and later directs his powers through it to attack the sisters in the attic.