Skimming is a generic term referring to taking a little bit off the top. In dairy, it refers to the cream at the top of the milk pail. With painting, it refers to a very thin coat of paint to identify imperfections with the wallboard. In business, it means taking a little bit of the revenue without recording the revenue to the books of record. It is very common in your cash based operations.
Skimming is a process in high cash based businesses whereby the owner takes cash from his till box and doesn’t record this cash as revenue on the books. The process is not illegal, but the failure to properly report the income when filing taxes is the actual problem.