What is a magic number, and why is it bad?

Question

What is a magic number?Why should it be avoided?Are there cases where it's appropriate?

Answer 1

A 'magic number' is some number that you have in your code with no particular reason other than 'it doesn't work right unless I divide by three here...'

The problem is that it usually implies a lack of completely understanding the process/algorithm/formula that you're working with. It has to have some 'magic' number in order to work, and the reason why isn't known.