Liquefaction of fully saturated sediments subjected to earthquake motion is often defined by the condition when effective stress reaches zero. On the other hand, the gradient of excess pore‐water pressure represents the net local surface force, which plays a crucial mechanical role for liquefaction. Herein, we consider two definitions of liquefaction, which yield different liquefaction initiation times and depths. Two hypothetical cases of liquefaction, earthquake‐induced and tsunami‐induced, are given to highlight the differences in the definitions of liquefaction. The results from the hypothetical cases show that the effective stress definition of liquefaction is a special case of the pore‐water pressure gradient definition of liquefaction.