Magic is a set of effects or abilities which can alter objects and forces. Generally, magic includes some of the following characteristics: it can act at a distance; it is to some extent employed directly by ellogous creatures without mechanical aid (except perhaps that of reagents which do not necessarily come into contact with the object affected); it has a variety of effects; it can be invested in objects for later triggering. Many physical forces and phenomena fit one or two of these criteria, but not enough to really be considered magic. Thus while Xivan magnetism, for instance, does act at a distance, since it isn't usable directly by ellogous beings and produces only a single type of effect (a magnetic force), it doesn't really fit the concept of magic and isn't so considered. Still, there is occasionally some subjectivity in the definition.
A practitioner of magic is called a mage. This is a general term that applies to any form of magic; more specific terms exist for particular specialties or magical systems.
In principle, magic may be capable of just about anything. However, in practice, some effects are more common than others, perhaps because they are the first effects that people try to produce. Among the most common effects are flight and invisibility; transformations of various sorts are also fairly widespread, mostly transformation of one creature into another, but also, slightly less commonly, of inanimate objects, or even transformation of substance, such as agalmatation. Other frequent applications of magic include thelxis, healing, and illusions.
Magical effects may take place immediately, producing their results and then leaving no lingering magical residue. This is often the case, for example, with healing magics, as well as with many magics that create fire or physical objects. It's also possible, however, for a magical effect to have a prolonged duration, or even to persist indefinitely. Such effects are known as enchantments, and are further categorized according to their extent, their object, and their means of activation.
One of the most common ways to invoke magic is through a spell—a magical effect consciously produced by a living entity, using some sort of ritual or mental activity to call upon the magic of the cosmos or some magic inherently within. Not every effect produced by living beings is considered a spell; the term is generally limited to those that can be done as discrete times (rather than continuous effects that must be somehow maintained), that can be reliably reproduced by anyone who knows how to invoke them (or anyone with magical ability, if this is limited to certain individuals), and that are more the product of words or thoughts than of physical action. Similar effects that draw upon some inherent magical ability in a particular creature are also sometimes called spells, but more properly are known as powers. Other methods of producing magical effects may exist, and are known collectively as paracarminical magic.
In addition to those magical effects that are produced by conscious invocation, magical enchantments may arise spontaneously under various circumstances, or be inherent in certain areas. The details of this, however, vary widely by the local workings of magic. Magical enchantments may also be placed into objects to be called upon later; such an enchanted object is known as a talisman.
Different forms of magic
The way that magic works is not a universal constant that works across all cosmoi, or even consistently within a cosmos. Multiple systems of magic are known, sometimes coexisting within the same world. Each distinct form of magic is called an arcanum.
Arcana vary widely in virtually all particulars. Some effects might be much more easily attainable in one arcanum than another. Certain types of enchantment may be easier; some kinds of objects may be affected more readily than others. Entirely different methods may be necessary to cast spells; in some arcana, spells may require magic words, in others physical reagents, and in others neither. In some arcana, anyone can cast spells if they learn the formulae; while in others, only certain people are born with magical ability—which may or may not be hereditary, or depend on when or where the individuals are born or a host of other factors. Paracarminical magics also vary widely between arcana; the geasa of dream magic, for instance, have no effect in void magic, and the bindings of the latter have no close analogue in the former.
Magic and science
Though magic is sometimes thought of as going against physical laws, in fact this is a simplistic viewpoint. Physical laws, by definition, encompass everything that happens, or can happen; if magic seems to contradict physical laws, then there is something about the physical laws that isn't understood. It is true, however, on some worlds that magic is incompatible with certain physical phenomena. It is common on some such worlds to speak of magic and technology being in opposition, or magic and science, but this isn't strictly accurate. Technology is, after all, the application of physical principles to create useful devices, and the fact that some physical principles don't work in the presence of magic doesn't mean that others aren't possible. In fact, the creation of talismans could be considered a form of technology, though it's seldom actually so called.
Similarly, the fact that certain physical principles do not apply in the presence of magic does not mean that science doesn't work. Science does not comprise any set of physical laws; science, properly, is rather a methodology of study and gaining knowledge, and is as applicable to magic as it is to anything else. Indeed, mages who experiment with different ways of casting spells, alchemists who make trials of mixing different reagents to see what happens, and enchanters who test the joint effects of various combinations of enchantments are all engaging in a form of science. There is even a term for the study of magic, which is called celemology.
(It may not be entirely inconceivable that on some world, the very fact of scientific investigation makes most magic impossible, the mental processes involved constituting some sort of antimagic field. However, since this would prevent any systematic study of the workings of magic, it's hard to see how magic could ever become well understood or much used in such a world.)
Still, in many words of advanced technology, people are reluctant to admit the possible existence of magic, to the extent that when something that fits the definition of magic does clearly exist, they call it by another name. "Psionics" is a common term used in such worlds to refer to something that by any broad standard could be considered magic.