This Recommendation contains a listing of the terms that are broadly used in the Recommendations associated with optical transport networks (OTNs). In the previous editions@ [ITU-T G.870] collected and defined major OTN terms. In this edition@ the Recommendations that originally introduced and primarily describe those OTN terms are identified as the Source Recommendation. From this revision@ the Source Recommendations are primarily responsible for the definitions of those terms. Official definitions are now provided by those Source Recommendations. For those OTN terms defined in the previous edition@ this edition of [ITU-T G.870] provides the pointers to the Source Recommendations as a directory. Some terms that were defined in the previous editions are now widely used in a general sense and there is no identified need to define them. Some terms are obsolete and are not recommended to be used. Abbreviations and acronyms@ which the previous editions listed@ are not included in this edition. It should be noted that the terms contained in this edition are based on the previous edition and are not exhaustive in OTN terminology. Directory document and/or service that indicate(s) the source Recommendations per OTN term are under study. OTN terms specific to synchronization or physical layer characteristics are not a part of this Recommendation.