That depends on the size of your company, but for AA/AAA type games, this is usually a so-called "Vertical slice". It's a demo that shows off a small piece of a game working fully (so demonstrating main systems, UI, etc.) that is then used to pitch full development to publishers.

It's rought equivalent to a pilot for a TV series: https://askagamedev.tumblr.com/post/77406994278/game-develop...

EDIT: I just remembered the other alternative - Early Access. This allows the developers to sell the game before it's complete and solicit gamer testing/feedback during the development process while iterating. It's a popular approach for smaller studios.