Magic is a mysterious, fundamental force that exists in the natural state of the world.
Unlike the other fundamental forces— gravity, electromagnetism, and so on— magic was not always a part of our reality. It was introduced to us during the Eclipse of the 21st century, and has since fundamentally altered the laws of physics in our world. It can be described mathematically as a field, where the values for spacetime are replaced with corresponding values in "realtime" (reality-time). In other words, magic contains energy, possesses momentum, and can be charted over time— but it does not always exist as a physical entity.
Magic has the unique property of responding to action potential, the electrical pulse of cell membranes in living things such as animals, plants, and fungi. Most importantly, it responds to the action potential in the synapses of the human brain. Humans who are in the right state of mind can influence magic to manifest in the physical world, channeling the energy stored in the magical field to influence reality. Humans who have learned to manipulate magic without assistance are known as witches. Otherwise, forces exist within magic that can give humans a push in the right direction— these "assisted" magic-users fall under their own classifications.
After the Eclipse, magic became the most important scientific discovery in human history.
In humans, magic is fiercely individualistic. Each person has a different way of thinking, and therefore each person manifests magic in different ways. Magic cooperates between manifestations of the same type, and it clashes when two or more different types come into contact. You can liken it to ripples on the surface of water: Ripples of exactly the same size and direction will merge, while ripples of different sizes and directions will disrupt each other. These principles create the foundations of magic-on-magic combat.
When magic isn't being manifested outside of the body, it can still manifest on the inside. This can have a wide variety of knock-on effects, with Shift being the best example. Humans who have been exposed to magic will find their bodies changing drastically to suit their needs; fighters will become stronger and sturdier, scholars will become more focused, and scouts will have more finely tuned senses. At the extreme, internal manifestations can completely transform the body and mind. Sometimes, this is the desired effect.
Magic can also be concentrated in the physical space, "sticking" it to the material world. Magical plants may produce it as a matter of course, or it can be extracted right out of the magical field. Magic-users who work with concentrated magic are known as alchemists. They tend to specialize in medical care, creating tonics and remedies that far exceed the limits of modern medicine. Different extracts have different effects, so sourcing the correct ingredients is extremely important— especially when magic has only been affecting the world for a decade or two.
These are just some of magic's applications. In magic communities, it's encouraged to search for your own solutions, experiment with your own abilities. Doing so could discover even new ways to use it to our advantage.
Magic is measured across the scope of reality, not the scope of physical space. Invisible and intangible is its default state. When magic exists as a physical quantity, it has been "manifested." These manifestations come in all shapes, scopes, and sizes— from tiny warbles in the electromagnetic field to enormous magical creatures. However, just measuring magic where it manifests is not enough. A mathematical approach requires charting the shift in magic's presence in reality over time: Realtime.
Realtime fields are measured in three dimensions: Time on the X-axis, saturation on the Y-axis, and distance on the Z-axis. Vectors are used to describe the movement of magic in the realtime space. These measurements are critically important for tracking the way that magic is influencing our reality; if left unchecked, it could begin to manifest uncontrollably, taking the laws of physics along with it. A healthy amount of release in the magical field prevents oversaturation and undersaturation events from damaging our world.
Currently, magic is becoming more saturated with each passing year.