Despite its appearance, the athame is a tool, not a weapon. And it's probably the most important of the four main tools used in Wiccan ceremonies, the other three being the wand, the pentacle, and the cup or chalice.
 
The main use of the athame is to channel and direct psychic energy. It is used for drawing the boundary line of the magic circle, the sacred and protected space in which rituals and spells are performed.
 
The athame usually has a double-edged blade to allow for more fluid movements of the hand and wrist when drawing invoking pentagrams, but by tradition, is never actually used for physical cutting.