A visitor begins a session on your home page, plays a video, and leaves. Does Google Analytics count that session as a bounce?
If you have used the default YouTube embed, the session will still count as a bounce. This is because Google Analytics records a visitor interaction only when it generates a hit.
If you have instead attached a Google Analytics event to a YouTube Player API embed, a video play will eliminate a bounce, since it will send a hit to the Google Analytics servers.
The same is true for a PDF download that you’re tracking as a virtual pageview, a Tweet link that you’re tracking as a social interaction, or a tracked Ecommerce transaction – all types of hits avoid a bounce for the session.
As a note, you can define an event as non-interaction. For instance, if a video begins automatically after 15 seconds on a page, you could still capture the video play as an event but opt to set the non-interaction parameter to true so that the video play in itself would not eliminate the bounce. For most events that you capture, however, a default interaction event is suitable.
In any case, it’s important to record all significant user interactions as some form of hit so your bounce rate and your overall Google Analytics data more accurately reflect user engagement.