Definition: characterized by straight lines
The word seems to be heavily used in architecture and engineering.
Why I Looked It Up
From a book about building skyscrapers:
The earliest skyscraper designers saw merit in highlighting the rectilinear nature of [the new rigid-frame system] on the building’s exterior, but by the second decade of the century, frames were increasingly hidden.
The frame of a skyscraper has to be rectilinear, because straight lines and specific angles are what transfer the loads and make the system work. The debate discussed above was how much of that “rectilinear-ism” should be displayed on the outside of the building.