A Mirror is an object that reflects light while preserving the original details of the light, thus creating a mirror image.
Mirrors can be powerful magical tools to those with the knowledge and power to use them. Mirrors can serve as portals between worlds and can be used to observe others and communicate with them. They can also be used to foresee the future.
A powerful sorceress named Kali was cursed into her own dimension and could only appear in reflections. She manipulated a girl named Aviva and was later able to possess her through the mirror. She was banished by the Charmed Ones when her reflection in the mirror was shattered.
The Wicked Witch was trapped in a magical mirror and kept in the Fairy Tales Fortress. When the mirror fell and shattered, she was freed and killed the keeper. She proceeded to trap his apprentice in the mirror and used it to spy on the Charmed Ones. When Piper vanquished the witch, the apprentice was freed.
Gideon used a magical mirror to communicate with his evil counterpart in the parallel world. It was later used by the sisters and Leo to work with their counterparts to stop and vanquish Gideon. When Gideon was vanquished, Leo destroyed the mirror to keep the worlds from ever crossing again.