The Book of Shadows is a Witch's magical diary filled with rituals, magical practices, spells, ethics, and philosophies that make up the beliefs and practices of that particular Witch. It is personal to every practicing Witch, and is unique to each of their own way.

In Medieval times, ones who practiced Magic and was accused of Witchcraft would be harshly persecuted. Thus, making the Witches refer to their magical book as the Book of Shadows-- implying that their workings and beliefs had to be done in secret.
Witches also encoded their magical recipes and rituals in a special alphabet known as Theban, to protect their book from prying eyes.