Preview

A preview is a small piece of something used to show what the whole is like before it is generally available. Typically, previews exist for movies and possibly video games. They usually are a small portion of video showing what the game or movie is like to advertise it.

"Show preview" button in MediaWiki

In software, a preview can be a feature that allows a user to see what a document looks like before the user prints it out, or posts it on a website.