Although romantic chemistry is often referred to in popular culture in Western societies, usually to describe an intense connection between two people, it has received limited attention in academic research. One limitation in previous research and a barrier to future research is the lack of a comprehensive definition of romantic chemistry. In the current study (N = 200 single and partnered people [88 women, 110 men, 2 non-binary participants], age range = 21-76), we examined the common categories in laypeople's definitions of romantic chemistry and assessed if people differed in how they described romantic chemistry based on demographic factors (i.e., gender, relationship status, relationship length, age, parental status). Two coders conducted a content analysis of participants' open-ended definitions of romantic chemistry using an inductive coding strategy. We found that people described romantic chemistry using nine distinct categories (from most to least common): positive interaction, mutuality, comfort, compatibility, similarity, unexplainable spark, sexual attraction, intense fixation, and physiological response. Largely, participants did not differ in how they defined romantic chemistry based on demographic factors. Our findings provide a comprehensive description of romantic chemistry that can assist the development of a measure of romantic chemistry and ultimately test the role of perceived chemistry in relationship initiation and maintenance.