Final answer:
A cost overrun represents the additional cost incurred beyond the estimated expenses of a project, while budget deficit refers to a government's expenditures surpassing its revenues in a fiscal year.
Step-by-step explanation:
The additional percentage or dollar amount by which actual costs exceed estimates is known as a Cost Overrun. This term is used to describe a situation where the estimated cost of a project is surpassed by the actual expenses incurred. The concept of cost overrun is different from a budget deficit, which refers to when a government's spending exceeds its revenues over a fiscal year. A cost variance meanwhile generally refers to the difference between estimated and actual costs, but can represent either an overrun or underrun. An expense surplus would not be an appropriate term in this context as it would imply that costs were less than estimated, thus resulting in a surplus instead of an excess.