Magic refers to various supernatural phenomenon which happen due to forces that are against the Laws of Science. It was often used to describe unusual natural phenomena before the Age of Enlightenment.
Magic in Fiction
Magic is, of course, a key component in fantasy and manifests itself in numerous ways.
Magic occasionally crops up in science fiction, generally hiding behind the name of psionics. A lot of science fiction technology does violate natural laws (such as FTL travel), but this is generally presented as the result of scientific principles which modern civilization has yet to discover. An analogous scenario would be a Victorian gentleman hearing that in a hundred years, men would be able to destroy cities by inducing nuclear fission in superheavy elements.
Magic in the MCU
In the Marvel Cinematic Universe, magic is energy drawn from alternate dimensions and used to create effects in our dimension. The effects that can be produced vary depending on the dimension used as an energy source.