The term "heart stopper" typically refers to something that causes a sudden and intense emotional or physical reaction. If you're looking for synonyms, there are quite a few options to choose from depending on the specific context. Some common alternatives include "showstopper," "jaw-dropper," "breath-taker," "stunner," "game-changer," and "mind-blower." Each of these terms conveys a similar sense of awe or shock, but with slightly different connotations or nuances. For example, "game-changer" may be more suited to a situation where a sudden turn of events alters the course of something, whereas "stunner" might refer more to a physical appearance or presence that leaves a lasting impression.