In programming, a "magic number" is a value that should be given a symbolic name, but was instead slipped into the code as a literal, usually in more than one place.
It's bad for the same reason SPOT (Single Point of Truth) is good: If you wanted to change this constant later, you would have to hunt through your code to find every instance. It is also bad because it might not be clear to other programmers what this number represents, hence the "magic".
People sometimes take magic number elimination further, by moving these constants into separate files to act as configuration. This is sometimes helpful, but can also create more complexity than it's worth.
