the core idea / concept of a layout
In architecture, the parti is the central idea / concept of a project. Can you reduce the building down to its most important structural aspects in 1 sentence?
Similarly, level design benefits from defining a central core idea that anchors the rest of the design. If you're able to pitch your level with a clear parti statement or sketch, then your collaborators (or even the player) will understand your intent better.

Parti in architecture

plan, section, and photo of Price Tower, designed by Frank Lloyd Wright (images from
Price Tower, "The Tree that Escaped the Crowded Forest"
section: central "trunk" core (elevators) that "branch" out for each floor
plan: a square with a cross in the middle