Navigating Classical and Nonclassical Realms of Nucleation and Crystallization

Our understanding of the formation and growth of solids from solution has shaped science for centuries. The classical nucleation and crystal growth theories provided a foundational framework, despite their necessary simplifications. Recent nonclassical concepts have challenged this canon and refined the general models of the molecular and mesoscale processes underlying solid-state genesis. This shift of paradigms triggered novel terminology, often coined ad hoc to describe unexpected observations, but also created ambiguity across disciplinary boundaries. Here, we review current terminology, summarize the perceived classical canon, clarify ambiguous concepts, correct recurring misconceptions, and highlight terms that require further specification. We draw connections between colloidal, molecular, and mineral systems to develop generalizing concepts, while emphasizing where analogous phenomenology arises from distinct mechanistic or stabilizing conditions. We further identify the Szilard postulate-that crystallization proceeds by addition of individual ions or molecules-as a key criterion for distinguishing classical from nonclassical nucleation and growth mechanisms. By clarifying terminology and conceptual boundaries, this review provides a reference framework and a field guide for current debates on classical and nonclassical nucleation and crystallization.