Magic is defined as the ability to control matter, in all aspects. Unicorns and Alicorns can use magic. Unicorn magic is usually very personal to the caster, and it is said that a unicorn can only learn one kind of magic. Twilight Sparkle is an exception to this rule, because her Unicorn Magic focuses on ALL magics, by contrast to Rarity's magic, which focuses on finding gemstones. Despite this, all unicorns can master Telekinesis along with their personal magic. Alicorns on the other hand are able to control magic on a much grander scale. Princess Celestia and Princess Luna are known to use their magic to move the Sun and the Moon respectively. It is unknown exactly how much magic they know and are capable of, but it is inferred that they know a wide range of very powerful spells.