Identity fluidity, the capacity for self-definition and group membership to shift across contexts, over time, and within category boundaries, has gained increasing visibility in public discourse, particularly in domains such as sexuality, gender, and race. This entry situates identity fluidity within a broader conceptual landscape, distinguishing it from related constructs like identity flexibility, complexity, and frame-switching. Drawing on the Social Identity Approach, we examine psychological mechanisms such as individual mobility, social creativity, context-driven salience shifts, and prototype alignment, alongside the consequences of fluidity for individuals, relationships, and societies. We identify conceptual, methodological, and sociopolitical challenges in applying social identity theory (SIT) and self-categorization theory (SCT) to fluid identities, especially in explaining category innovation and intersecting identities. Future directions include mapping how fluid identities are created and institutionalized, analyzing fluidity as a form of social movement and minority influence, and assessing its psychological and societal consequences. This integrative agenda advances a multi-level understanding of why and how identity boundaries change and with what effects.