Parametric modeling refers to the relationships among all elements in a project that enable the coordination and change management that Revit provides. These relationships are created either automatically by the software or by you as you work.
In mathematics and mechanical CAD, the numbers or characteristics that define these kinds of relationships are called parameters; hence, the operation of the software is parametric. This capability delivers the fundamental coordination and productivity benefits of Revit: change anything at any time anywhere in the project, and Revit coordinates that change through the entire project.
Revit immediately determines what is affected by changes and reflects those changes to any affected elements. A fundamental characteristic of Revit is the ability to coordinate changes and maintain consistency at all times. You do not have to intervene to update drawings or other content. When you change something, Revit uses 2 key concepts that make it especially powerful and easy to use. The first is the capturing of relationships while the designer works. The second is its approach to propagating building changes. The result of these concepts is software that works like you do, without requiring entry of data that is unimportant to your design.