This research is focussed on understanding what urban water security means -a surprisingly elusive concept given the global shift from rural to urban living. We first make the case for a distinct urban water security definition. We then identify 25 unique water security definitions, of which three relate to the urban context but all with scope for improvement. Applying novel indices, we assess the prevalence, complexity and evolution of themes and dimensions within all definitions and find a stable spectrum of themes; but note a shifting emphasis towards environmental and social dimensions, away from quality and quantity of supply. Overall the definitions are becoming more comprehensive by simply listing more outcomes to be achieved. Instead of this 'shopping-list' approach, we propose a simplified urban water security definition with a focus on agreement of needs with community stakeholders, while using the themes to guide what the objectives might be.