I tend to think of "magic" as meaning fragile, indirect coupling; for example "magic numbers" at the start of files, which are only meaningful if the consuming application has a corresponding pattern in its database.

Or sets of things which only work if given in a particular order. Or unrelated functionalities which interfere with each other due to implementation details. And so on.

In other words, leaky abstractions where thinking in terms of the concept has so many edge cases that it's easier to think in terms of the implementation instead. Or conventions which can only be used via memorisation, trial and error, stack overflow and reading the source (often this is due to shoehorning things into language models which aren't amenable; e.g. all of the things aspect oriented programming tries to do)