This part of IEC 60118 gives definitions for common hearing aid features such as noise reduction or feedback reduction, etc. Only acoustical inputs are considered. Binaural features are currently not covered in this document. In addition, measurement procedures are described to verify hearing aid features. The objective is not to evaluate the performance of features but to verify their existence and functionality. Furthermore, definitions and procedures are kept as general as possible so that this document can be applied to various types of hearing aids, e.g. air conduction hearing aids or bone conduction hearing aids. To this end, the general definition for hearing aid of IEC 60118-0:2015 is adopted, and this document does not refer to any specific ear simulator or acoustic coupler but uses a general definition of a coupler. However, if a general view is not applicable or leads to unclear or complex wording, the situation for an air conduction hearing aid is considered, only. Nevertheless, in Clause 4, an explanation is given on how this document can be applied to hearing aids which do not use air conduction.