What is Accounting Method?

Legal Definition
In the field of analysis of algorithms in computer science, the accounting method is a method of amortized analysis based on accounting. The accounting method often gives a more intuitive account of the amortized cost of an operation than either aggregate analysis or the potential method. Note, however, that this does not guarantee such analysis will be immediately obvious; often, choosing the correct parameters for the accounting method requires as much knowledge of the problem and the complexity bounds one is attempting to prove as the other two methods.

The accounting method is most naturally suited for proving an O(1) bound on time. The method as explained here is for proving such a bound.
-- Wikipedia
Legal Definition
The rules that tell when and how both income and expenses are reported. The two common ways are accrual and cash method.