A parti pris is an organizing thought or decision behind an architect's design, presented in the form of a basic diagram or a simple statement. It may be shortened to “parti.” The term comes from 15th century French, in which “parti pris” meant “decision taken.” Later, it took on the meaning of “bias” or “prejudice.” The development of the parti frequently precedes the development of plan, section, and elevation diagrams.
Description sourced from Wikipedia.