pre-roll also preroll

a video advertisement that appears immediately before an online video

If you're a regular viewer of online videos, then you'll certainly be familiar with the experience of having to twiddle your thumbs for a few seconds whilst some kind of advertisement plays out before you can get to the thing you're really interested in. What you may not know however, is that there's a name for this type of ad — it's called a pre-roll.

pre-roll advertisements have become much more common in recent years, possibly because they're assuming greater significance in an era when TV viewers are increasingly skipping advertising slots

A pre-roll is a promotional video that plays before the main video content the user has selected. In the unlikely event that you haven't seen one, then what happens is this: you click the play button on an online video of your choice and, instead of the thing you want to watch appearing, you're first presented with a video advertisement for a car, or home insurance, or the latest detergent, or … the possibilities are endless. A few moments later, after a pre-determined period of advertising has elapsed, you're finally able to watch what you were interested in.