Final answer:
The appropriate term for the situation where the hypothetical tax calculated is less than the actual income tax of an employee working overseas is an 'underpayment of tax'.
Step-by-step explanation:
In the scenario where Josh, a civil engineer at a U.S. construction company, has been working overseas for the last three years, and the hypothetical tax calculated by his company turned out to be less than his actual income tax, the appropriate term for this situation is an underpayment of tax. Josh will likely be required to pay the difference between the hypothetical tax and the actual tax. If the hypothetical tax was meant to estimate his U.S. tax liability while working abroad, it's possible that adjustments will need to be made either through an additional payment to the IRS or through future tax withholdings to reconcile the underpayment.