Spells are constructs created by magicians. Due to the property of magic requiring a specific level of "brute-forcing" to work, a large amount of unnecessary energy is consumed. Spells reduce the level of excess energy needed, improving efficiency as they are expanded upon. They are reusable, and magicians are able to use more than one spell.
Notable Spellcasters Edit
Due to the fact that many illusions, such as those hiding the secret passages in the castle, are time-consuming and draining to create, it is safe to assume that Otto uses spells to formulate his most commonly summoned attacks.
As Emelles is only ever shown using his abilities well to shapeshift between human and raven form, it is highly likely that he uses a highly-perfected spell to allow him to smoothly change forms.
Lucina's powers are implied as being extremely costly to use. Therefore, her more proficient use of her time slowing/stopping magic as time passes makes it greatly possible that she has been creating a spell that she uses to call upon her powers.