The SABRE (Synergetic Air-Breathing Rocket Engine) is a combined-cycle propulsion system that incorporates an air-breathing mode (similar in principle to ramjet/air-breathing technologies) alongside a closed-cycle rocket mode for transatmospheric flight.
The retrieved literature consistently characterizes the SABRE engine as a combined-cycle propulsion system featuring an air-breathing mode used during the atmospheric phase of flight prior to switching to pure rocket mode.