The use of the term 'net zero' has rapidly and recently become mainstream but is often not well-defined in the literature. A brief history of the term was researched, followed by a systematic literature review to consider the research question: how have the different net-zero terms been defined in the literature, and do they indicate knowledge or process gaps which identify future research opportunities? Academic research articles were searched for the term 'net zero' and filtered for the term

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... definition', resulting in 65 articles. Definitions were analysed according to scale: single-building, community, urban-system, and country-wide scale. The search did not return any definitions concerning country-wide emissions (from agriculture, forestry, large-scale transportation, or industrial and mining processes), a surprising outcome given the emissions impact of these areas. The main knowledge and process gaps were found to be in four areas: governance, design, measurement and verification, and circular framework. A clear net-zero definition is required at the appropriate scale (single-building or urban-system scale), which includes explicit system boundaries and emission scopes, life-cycle energy and greenhouse gas (GHG) emissions and should incorporate a dynamic approach. The scale most likely to achieve net zero is the urban-system scale due to the potential synergies of its interacting elements and energy flows.