(1) In communications, the time required to reverse the direction from sending to receiving or from receiving to sending on a communications line.
(2) The elapsed time between entry of the first character of the first input into the input interface and the passage of the last character of the last output through the output interface.
(3) The total time consumed from the start to the completion of a specific unit of work measured at specific interfaces. When multiple inputs and/or multiple outputs are parts of one unit of work, intermediate turnaround time specifications may be needed.