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The design diving speed. It is the highest speed at which the aircraft is allowed to fly. It is one of the speeds used in establishing the structural strength of the aircraft.
The rate of descent, or negative, vertical speed. It is not the IAS (indicated air speed) or the TAS (true air speed) but the vertical speed. It is measured by a vertical speed indicator, or VSI, or, more accurately, by noting an altimeter difference in a certain period of time. See vertical speed indicator.