This article seeks to clarify the notion of the “medium-sized city” by articulating three dimensions - size, functions, and representations - on the basis of a wide-ranging scientific, institutional, and media corpus spanning more than half a century. It shows that this category, although longstanding and widely mobilized, remains marked by a persistent vagueness: demographic thresholds vary across sources and periods - most often between 20,000 and 100,000 inhabitants, sometimes extended to 200,000 - reflecting not only statistical choices but also political and media uses. The article further interrogates the functions traditionally assigned to medium-sized cities. Portrayed as relay cities, they are said to ensure a double connectivity between hinterlands and metropolitan areas; understood as receiving cities, they would accommodate populations leaving rural areas; seen as stepping-stone cities, they would offer a temporary stage in social and residential mobility trajectories. The analysis demonstrates that these functions, inherited from the territorial planning policies of the 1970s, shed light on certain dynamics but fall short of capturing the diversity of local trajectories. The article also examines the weight of representations. The myths of the “human-scale city” and the “city where life is good”, forged in the context of the 1970s, continue to shape political, media, and communication discourses. It shows that such valorizing images contribute to the legitimization of the category while often obscuring less flattering realities such as deindustrialization, impoverishment, or urban sprawl. Ultimately, the medium-sized city emerges as a hybrid construction, more political than scientific. The article proposes using it in a reflexive way - not to settle on a single definition, but to interpret territorial dynamics and to guide differentiated policies, adjusted to the specific resources and vulnerabilities of each context.