Full Idea
A categorical imperative would be one which represented an action as objectively necessary in itself apart from its relation to a further end.
Gist of Idea
A categorical imperative sees an action as necessary purely for its own sake
Source
Immanuel Kant (Groundwork of the Metaphysic of Morals [1785], 414.39)
Book Reference
Kant,Immanuel: 'The Moral Law (Groundwork of Morals)', ed/tr. Paton,H.J. [Hutchinson 1948], p.78