Sometimes called "Sorcery", Magic is a term used to describe the mystical ability of some characters to use their ethereal abilities to cast spells, conjure magics and create a large number of other abilities from it. It differs from Occultism because these characters have an inherit use of their abilities which, in most cases, extend from themselves.
There are many uses for magic and because of its mystical nature many abilities may seem like magic or be considered magic such as the powers of the Spectre or as some mistake a Green Lantern Ring to have magical properties.
To cast a spell, the sorcerer requires a source of magical energy. These sources can include internal reserves, mystical beings, alternate dimensions, or from their surroundings, supposedly from thin air; the dimension of the Earth was once full of these energies. With the destruction of Atlantis, and various pagan holy sites, these energies were displaced, allowing for scientific advancement. Some places are still strong in mana, such as Stonehenge, Parliament of Trees and Arkham Asylum, and magical dimensions such as Azarath and Gemworld.
Magic and science are opposing forces. The Guardians of the Universe gathered the aleatory magical energies from Oa into the Starheart, and expelled it from the planet; therefore allowing that science could develop on Oa.
In magic, nothing is free: The price of magic always involves suffering. Zatara's price was his lengthy and painful separation of his daughter; John Constantine's price has included various failures and the deaths of his friends; and Amethyst's price was eventual blindness and the loss of her family.