Positive Temperature Coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Components which have useful engineering applications represent a rapid increase in resistance with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase.
This effect can be used to create resettable fuses, often as part of a semiconductor. As the temperature rises, nearing danger levels, the resistance increases, so less current flows, thereby protecting the vulnerable equipment that receives the current flow.