The common families are miscible or immiscible gas injection, chemical flooding with polymer, surfactant, or alkali, and thermal methods such as steam. They try to change the residual saturation, the mobility ratio, or the oil viscosity, rather than only to hold pressure.
Usage is not uniform. Some operators call any injection enhanced recovery. Most reservoir engineers reserve the term for processes beyond a waterflood, and call the waterflood itself secondary recovery. A project sold as enhanced recovery still has to beat the cost of the injectant with the extra oil it produces.