ACIS header keyword whose value represents the amount of time required to flush a CCD before a frame of data is obtained. A non-zero preflush is required if the value of EXPTIME is smaller than the time required to read-out one frame of data. In this case, some time is spent flushing the exposed region of the CCD to avoid transferring charge into the frame store region before the frame store has been entirely read out. Events that occur during a preflush are discarded onboard. Therefore flushes are effectively "dead time." Under most circumstances, the value of FLSHTIME is 0.0 s. If an observation uses interleaved mode then there are separate values of FLSHTIME for the short and long frames (e.g. 3.17032 s and 0.0 s, respectively).
The observation times entry and the chandra times help file explain how the various exposure keywords are related.