The term duration (also known by the abbreviations DU or DUR) refers, in a big picture sense, to the total and complete amount of time that will pass from the moment that a project is initiated until the very last moment in which the final deliverable is provided to the buyer and those final deliverables are approved. The duration, more specifically, represents the total and complete sum total of work periods (not including any non working periods which can include holidays or other scheduled time of) that is required to complete a particular schedule activity or specific component of the work breakdown structure. Duration is often expressed in terms of work days or work weeks, and often times is confused with the similar but slightly different term elapsed time. Duration can be directly contrasted with the term effort. For more information, please see the definitions for original duration, actual duration, and remaining duration.
This term is defined in the 3rd and the 4th edition of the PMBOK.