An uno lamp shade is a type of shade that fits onto a lamp socket with an uno fitter. The uno fitter has arms radiating outward attached to the inside edges of the shade that support a hollow round piece that slides over the socket.
The central piece that fits onto the lamp socket is called a "fitter." The arms radiating outward from the fitter that attach to the shade are called the "spider."
Uno lampshades are available with threaded and non-threaded as well as plastic and metal fitters.
Floor lamps with an extended arm and a downward pointing socket traditionally use threaded uno fitters on their shades. Recently, many imported lamps with standard orientation are also using uno lamp shades.
Uno shades allow a shade to sit on a lamp without the added hardware of a harp or finial.
Uno lampshades are not necessarily interchangeable. The size of the opening, the length of the arms and the presence or absence of threads will all determine what shades and bases are compatible.
Threaded uno lampshade fittings wear down over time, making the shade sit shakily on the electrical socket. This poses a fire hazard and causes the lamp to tip over easily if the shade is bumped into.