An estimating method where the base costs of individual work items or resources are calculated into task, resource, and project cost totals.
(Wimincken.com PM Glossary)
Bottom Up Estimating – Overall estimates for the project are built up from the detailed level and aggregated to give totals for the project as a whole. The opposite of Top Down.
(Wideman Comparative Glossary of Common Project Management Terms v2.1 )