This Technical Report provides a coherent set of terms, definitions and symbols for concepts and physical quantities related to the overall energy performance of buildings and its components, including definitions of system boundaries, to be used in all standards elaborated within ISO on energy performance of buildings. These terms and definitions are applicable to energy calculations in accordance with this Technical Report and standards on the overall energy performance of buildings and their components, to provide input to this Technical Report or using output from this Technical Report. They are based on existing terms and definitions from standards and other documents referenced in the bibliography. NOTE Slightly different definitions might be applicable to other situations, e.g. design of installations.