That was the first definition offered up by google, but it misses by a mile. The term "proscenium" specifically refers to a theatre layout in which the stage has a rectangular frame known as the "proscenium arch". It has a front and a back, and unlike "theatre in the round", the audience has a limited and fixed view port into the performance.

Look at this page: It has an "arch" across the top--the header; it has an orchestra pit in the footer. The columns on each side correspond to stage left and stage right. And the central large arena is the stage.

proscenium: noun - A Web page design paradigm so universal that nobody notices it.

Tom's right that "Proscenium" as used here is my project. The Proscenium software started out as a user interface for the WordPress blogging software, so named because it produces this proscenium-shaped page layout. From that tiny acorn grew a mighty tail that wags the WordPress dog: Powering the Zone is just a hobby, in its day job Proscenium is running an online educational "platform" delivering dozens of courses to thousands of students. The Zone uses only a fraction of its features.

That's one mystery solved.