Top-down Cost Estimating is the preparation of a cost estimate by using judgment and experience to arrive at an overall total amount, usually done by an experienced estimator or manager making a subjective comparison of the project with similar previous projects.
(Wideman Comparative Glossary of Common Project Management Terms v2.1 )
Top-down Estimating – An analogous estimating method that uses the actual costs of a previous, similar project as the basis for estimating cost totals of a current project. This method of applying cost totals is often used when there is a limited amount of detailed information about the project (such as in early phases of the project). (Wimincken.com PM Glossary)
Top-down Estimating – See Parametric Cost Estimating .