A hinged case back has a solid hinged cover over the back of the watch case. Today, the internal case back often includes a sapphire window to view the movement, as on the Patek Philippe Calatrava 5227, though in the past it was often solid or entirely open.
This style is often also called an officer's case back in reference to the Officer's watches that typically feature them or a Hunter case back in reference to the Hunter pocket watch style. Patek Philippe refers to this embellishment as a hinged dust cover.