The central principle of Kantian ethics. According the the so-called "Universal Law Formulation" of the principle (Kant presented other formulations), the Categorical Imperative reads "Act only on that maxim whereby you can at the same time will that it should become a universal law." Cf. maxim
is the necessary and absolute moral law believed to be the ultimate rational foundation for all moral conduct. "So act that you can will the maxim (principle) of your action to be an universal law binding upon the will of every other rational person." Categorical imperatives are absolutely binding.
This is a philosopical term Kant made up. It has something to do with ethics. It's one of those things that somebody'll mention in philosophy classes but I don't really understand because I haven't had a class in ethics yet. It's a very imposing term, however, and it certainly sounds very confusing. Mentioning categorical imperatives is great fun at parties, especially when you want somebody to leave you alone. You just have to hope that they don't ask you what it means.