This Recommendation provides the definition for different logarithmic power level measurement units in current use in telecommunication systems. This Recommendation also provides the relationship amongst those units and usage examples. The text herein is a merger of information that has been previously dispersed over several publications and supersedes the contents of ITU-T Rec. B.12, of Annex A/G.100, of Annex A/G.101, of Annex B/Q.551, and of 3.8/G.101.