Source : WordNet®

rotational latency
     n : (computer science) the time it takes for a specific block of
         data on a data track to rotate around to the read/write
         head [syn: {latency}]

Source : Free On-Line Dictionary of Computing

rotational latency
     
        The time for the start of the required {sector} on a disk to
        appear underneath the read/write head.  The worst case is
        where it has just passed the head when the request is
        received.  For a {disk drive} with N heads per surface,
        rotating at R revolutions per minute, the average rotational
        latency will be
     
        	L = 30/NR seconds.
     
        Rotational latency is one component of {access time}.