Underestimation means to consider the lower figure of your
possible results whereas overestimation means to consider the
higher figures. For example, you are asked to estimate the number
of students in your class just by taking a glance. You are told to
underestimate. Say there are 29 students in your class, you
underestimate by stating there are about 25 or 20, depending if you
are estimating by 5's or 10's.