Political polarization (spelt polarisation in British English, Australian English, and New Zealand English) is the divergence of political attitudes away from the center, towards ideological extremes. Scholars distinguish between ideological polarization (differences between the policy positions) and affective polarization (an emotional dislike and distrust of political out-groups).

Most discussions of polarization in political science consider polarization in the context of political parties and democratic systems of government. In two-party systems, political polarization usually embodies the tension of its binary political ideologies and partisan identities. However, some political scientists assert that contemporary polarization depends less on policy differences on a left and right scale but increasingly on other divisions such as religious against secular, nationalist against globalist, traditional against modern, or rural against urban. Polarization is associated with the process of politicization.

Polarization in itself is typically understood as "a prominent division or conflict that forms between major groups in a society or political system and that is marked by the clustering and radicalisation of views and beliefs at two distant and antagonistic poles." as defined by the Institute for Integrated Transitions and Ford Foundation.

Political scientists typically distinguish between two levels of political polarization: elite and mass. "Elite polarization" focuses on the polarization of the political elites, like party organizers and elected officials. "Mass polarization" (or popular polarization) focuses on the polarization of the masses, most often the electorate or general public.

Elite polarization refers to polarization between the party-in-government and the party-in-opposition. Polarized political parties are internally cohesive, unified, programmatic, and ideologically distinct; they are typically found in a parliamentary system of democratic governance.

In a two-party system, a polarized legislature has two important characteristics: first, there is little-to-no ideological overlap between members of the two parties; and second, almost all conflict over legislation and policies is split across a broad ideological divide. This leads to a conflation of political parties and ideologies (i.e., Democrat and Republican become nearly perfect synonyms for liberal and conservative) and the collapse of an ideological center. However, using a cross-national design that covers 25 European countries, a recent study shows that it is not the number of parties itself, but the way a party interreacts with another that influences the magnitude and nature of affective polarization.

The vast majority of studies on elite polarization focus on legislative and deliberative bodies. For many years, political scientists measured polarization in the US by examining the ratings of party members published by interest groups, but now, most analyze roll-call voting patterns to investigate trends in party-line voting and party unity. Gentzkow, Shapiro, and Taddy used the text of the Congressional Record to document differences in speech patterns between Republicans and Democrats as a measure of polarization, finding a dramatic increase in polarized speech patterns starting in 1994.

Mass polarization, or popular polarization, occurs when an electorate's attitudes towards political issues, policies, celebrated figures, or other citizens are neatly divided along party lines. At the extreme, each camp questions the moral legitimacy of the other, viewing the opposing camp and its policies as an existential threat to their way of life or the nation as a whole.