A spell is a unit of magic use. It consists of three parts. Intention, Execution, and Effect.
Intention is what the user intends to accomplish by the spell, and is impacted by the user's grasp of will magic. Execution is the activation of the spell by various means, and is impacted by the user's grasp of knowledge magic. Effect is the actualization of what was intended by the user, and is impacted by the user's grasp of leyline magic.