This chapter examines key conceptual debates surrounding affective polarization in both the United States, where this field of research originated, and in comparative studies of multiparty systems. We begin by restating the dominant definition of affective polarization: the gap between positive feelings towards one's in-party and negative feelings towards out-parties. We distinguish between affective polarization and related concepts such as negative partisanship, partisan prejudice, democratic backsliding, and ideological polarization. For multiparty systems, the chapter discusses challenges in extending the concept, including how to conceptualize multiple in-parties and out-parties, the nature of partisanship outside the US and whether non-partisan identities based on ideologies or issues can also be objects of affective polarization. We also discuss the differences between party, voter, and leader affective polarization. We argue that, while complex, the concept of affective polarization has proven useful and versatile for understanding contemporary political conflicts across different contexts.