IEC 61666:2010 establishes general principles for the identification of terminals of objects within a system, applicable to all technical areas (for example mechanical engineering, electrical engineering, construction engineering, process engineering). They can be used for systems based on different technologies or for systems combining several technologies. This second edition cancels and replaces the first edition of IEC 61666 published in 1997 and edition constitutes a technical revision. It includes the following substantial changes with respect to the previous edition: - the terminology used in the publication has been adapted to the one used in IEC 81346-1 Ed. 2; - a more comprehensive description of the designation principles is provided; - additional examples illustrating terminal designations related to the function aspect and location aspect are provided; - an additional example illustrating the use of terminal designation sets is provided; - the former informative Annex A has been turned into a clause in the standard. It has the status of a horizontal standard in accordance with IEC Guide 108.