A parti is the central idea or concept of a building

A parti [par-TEE] can be expressed several ways but is most often expressed by a diagram depicting the general floor-plan organization of a building and, by implication, its experience and aesthetic sensibility. A parti diagram can describe massing, entrance, spatial hierarchy, site relationships, core location, interior circulation, public/private zoning, solidity/transparency, and many other concerns. The proportion of attention given to each factor varies from project to project.

[…] It is unlikely, if not impossible, to successfully carry a parti from an old project to a new project. The design process is the struggle to create a uniquely appropriate parti for a project.

Some will argue that an ideal parti is wholly inclusive—that it informs every aspect of a building from its overall configuration and structural system to the shape of the doorknobs. Others believe that a perfect parti is neither attainable or desirable.

– No. 15 from 101 Things I Learned in Architecture School by Matthew Fredrick

The Robie House designed by Frank Lloyd Wright; parti and photo