is an approximate calculation of something. For an account of statistical estimation, see estimator
The ability to accurractly estimate the time/cost taken for a project to come to its successful conclusion has been a serious problem for software engineers.
The use of repeatable, clearly defined and well understood software development process has in recent years shown itself to be the most effective method of gaining useful historical data that can be used for statistical estimation.
This is one of the reasons that the use of CMM, CMMI and SPICE process models have become so prevalent.