Prototype
Also said as Proof of concept
A quick, throwaway version built to answer one question, such as "is this fun?" or "can the engine do this?". You expect to bin it.
An idea on paper costs nothing and proves nothing. A few days of ugly work can save years spent building something that was never going to be fun.
Say it
“Before we plan anything, spend three days prototyping the grapple with grey cubes. I only want to know whether swinging is fun. Throw the code away afterwards.”
Where you’ve seen it
Nintendo first tried out Breath of the Wild's physics and chemistry in a flat 2D prototype drawn like the original Zelda, and showed it at GDC in 2017. Fire, wind and metal were already interacting.
Not to be confused with vertical slice
A prototype answers one question as cheaply as it can, and looks terrible. A vertical slice shows the finished quality of the whole game, and costs far more.