process of finding an estimate, or approximation, which is a value that is usable for some purpose even if input data may be incomplete, uncertain, or unstable
The English used in this article or section may not be easy for everybody to understand. (April 2012)
Estimation is the approximation of a result that one can use even if they are using information that is not clear or is incomplete. It is like making an educated, reasonable guess based on the information given.
If an estimate is more than the actual amount, then it is called an overestimate. If the estimate was less than the actual result, then it is called an underestimate. This is different from a prediction.