Idioms are traditionally defined as expressions whose meanings cannot be derived from their constituent words. This definition, however, is limited in that it focuses primarily on non-compositionality, overlooks fixed expressions characterized by structural oddity or partial semantic transparency, inadequately distinguishes idioms from other multi-word expressions, and fails to capture the gradient nature of idiomaticity. In response, this paper critically re-examines conventional definitions and proposes a more operational characterization of idioms as institutionalized expressions of two words or more, but fewer than a sentence, whose individual words cannot be replaced with other similar words, and that are either non-compositional or odd in structure, or both. Using a conceptual and critical review approach, the study develops a practical two-stage identification framework built around five criteria, namely multi-wordiness, institutionalization, restrictive flexibility, non-compositionality, and structural oddity. The framework provides a systematic procedure for distinguishing idioms from collocations, metaphors, and other multi-word expressions, thereby enhancing conceptual clarity in phraseological research and language pedagogy. Rather than treating idiomaticity as a binary property, the study conceptualizes it as a continuum shaped by multiple interacting criteria, reflecting the view that linguistic categories are not inherently fixed but are sensitive to context, use, and convention. This perspective offers researchers and language teachers a principled and practically applicable tool for identifying and analyzing idiomatic expressions.