Closely related to the motivation for potential functions.
Sometimes the worst-case running time of an operation does not accurately capture the worst-case running time of a sequence of operations.
The idea of amortization is that we will charge induhvidual operations with the average cost for all operations in the sequence.
So, if a sequence of 100 operations requires 1000 units of time, we'll say each operation has an amortized cost of 10 (even if one of them takes 901 units and the rest take unit time).
