There is a delay between from when the input crosses the reference voltage and the output responds. This is called the Propagation Delay. Propagation delay can be different between high-to low and low-to-high input transitions. This is shown as t_pLH and t_pHL in Figure 8-1 and is measured from the mid-point of the input to the midpoint of the output.

Figure 8-1 Comparator Timing Diagram