Estimating using an algorithm in which parameters that represent different attributes of the project are used to calculate project effort, cost, and/or duration. Parametric estimating is usually used in top-down Estimating.(AllPM.com)
Parametric Estimating – An estimating technique that uses a statistical relationship between historical data and other variables (e.g., square footage in construction, lines of code in software development) to calculate an estimate .
(Wideman Comparative Glossary of Common Project Management Terms v2.1)