Original duration is one of the most critical elements of effective and comprehensive project management, as it is a truly valid representation of the length of time that it was originally assumed it would take from the onset to the conclusion of the project. To be more specific, in terms of the life cycle of the project, the original duration (also known by the two letter anagram OD) refers specifically to the activity duration that was assigned at the onset of activity only. This original duration is a static element, and is not a living mark. It is not in any way, shape, or form updated as the project advances and there are some very particular reasons why it is a good idea for it to not change. Primarily, it is helpful for it to remain in its original state because it allows the project manager and project management team to view it in comparison to the actual duration and to the remaining duration when making an assessment of the progress to date.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.