Energy efficiency is a complex concept which is represented in diverse fields including engineering, economics, energy, computer sciences, environmental sciences, mathematics and physics. The social sciences literature on energy efficiency, however, remains significantly underrepresented, comprising just 2.6% of the total energy efficiency literature found in this study. Energy efficiency is an important energy policy strategy globally to reduce energy consumption, secure energy supply, and reduce greenhouse gas emissions. Despite this, however, evidence shows that on a global scale, energy demand is rising and climate mitigation targets are not being met. There is an acknowledgement by scientists that these challenges cannot be viewed as simply technical in nature but rather the product of collective social and cultural factors. Therefore, more social science research is needed to support an energy transition towards cleaner energy sources. Specifically, there is a need to further disentangle what is meant by energy efficiency from a social sciences perspective, including, critically, its conceptual foundations and practical applications. This review seeks to understand these issues by exploring how energy efficiency is conceptualized, both historically and today, by different actors. Research shows that the way that the concept of energy efficiency is applied to the physical, material world is a value judgement that brings with it societal trade-offs that are not fully understood. That is, that applying any given conceptualization or methodology of energy efficiency to physical processes can privilege certain interests over others, and affect society in different ways. For example, such tradeoffs include pollution displacement, lower than expected energy savings and an unfair cost burden on certain groups. Bringing together technical and qualitative insights from economics, energy, engineering, science and technology studies and history, this review builds on the work of efficiency and energy social scientists to illustrate what we need to do in order to bridge a conceptual gap in the energy efficiency literature, and in practice. The findings show a diversity of conceptualizations in the energy efficiency literature, highlighting the fact that energy efficiency can mean different things depending on how it is defined and applied. The review finds that greater efforts are needed to integrate energy efficiency discussions into sociological frames including ethics, equality, philosophy and history for more diverse, comprehensive and balanced research.