The term actual duration can refer to a couple of different things depending on usage and context. In one usage, the term actual duration can refer to the exact amount of time between the date on which the actual start date of a schedule activity falls and the date on which the project schedule’s data date falls (assuming the schedule activity is still in progress). Alternately, in the event that the schedule activity has concluded, the term actual duration can refer to the time between the actual start date of the schedule activity and the actual finish date. The actual duration can be measured in any standard calendar units, including days, weeks, months, years, etc., and the most appropriate should be chosen given the situation. The actual duration may result in findings very similar to the durations estimated at the start of the project, or due to factors that were unforeseen at the time (delays, financial hang-ups, etc.), the actual duration may vary wildly from the initial projections.
This term is defined in the 3rd and the 4th edition of the PMBOK.