Terms that indicate "pay attention, we're getting ready to change"

To Natalia's point about using fade-to-black and other subtle visual cues... I've also seen designers use a change in the player's view mode as a nice transition. Like, maybe they use Standard View for most of their content, but on slides where a new section starts, they might use Slide Only View, or No Sidebar View. It can be a nice reminder to the learner that they're entering a new portion of the course

In case anyone's not familiar with switching up the View Mode, here's a tutorial: http://community.articulate.com/tutorials/products/customizing-the-view-modes-of-your-player.aspx