A rocket nozzle design that allows combustion to occur around the periphery of a spike (or center plug). The thrust-producing, hot-exhaust flow is then shaped and adjusted by the ambient (atmospheric) pressure. It's also called a plug nozzle or a spike nozzle.
Liquid nozzles, such as those from fire hoses, are called converging because the area decreases along the length of the nozzle to increase the speed. Typical liquid nozzles have a simple conical shape and are designed to a specific ratio of inlet to outlet areas.
In the case of gas nozzles, the gas density can change dramatically from the pressure between the inlet and outlet of the nozzle. At very high gas speeds, this effect is so significant that the basic shape of the nozzle must change to a converging-diverging form. The diverging portion is necessary to accommodate the expansion of the gas as it accelerates to lower pressure.