Graybox
Also said as Blockout, Whitebox, Greybox
A playable version of a level or game made only of plain untextured shapes. The hero of this site, Noun, begins as one: a square with the physics values the code calls GRAYBOX.
Layout problems are cheap to fix while a level is boxes and expensive after artists have dressed it. Grey also stops testers commenting on the art, so they tell you about the space.
Say it
“Graybox all three arenas this week, with no textures. I want to play them and move the cover around before an artist touches anything.”
Where you’ve seen it
Valve's Source engine comes with plain "dev" grid textures in orange and grey, marked so you can read distances off them, for blocking out a map before any art exists.
Not to be confused with prototype
A prototype asks whether an idea is any good, and can be made of anything, even paper. A graybox is a real level in the real engine, without its art.