The object of this report is to describe and interpret various terms considered to be of basic importance to concepts and practical application in the design and evaluation of electromagnetically compatible systems. In addition, attention is drawn to the distinction between electromagnetic compatibility (EMC) tests carried out in a standardized set-up and those carried out at the location where a device (equipment or system) is installed (in situ tests). The terms and their definitions are given in clause 2, with reference to chapter 161 of the IEV [11. The application of the terms is described in clause 3 and an interpretation of their definitions is presented in the annexes.