Engagement : Definition
Any user interaction with your site or app.
For a content publisher, engagement may be scrolling slowly down the page, indicating the user is scrolling to read vs scrolling to find out length of article.
For an ecommerce site, engagement may be viewing product details, or spending a certain amount of time on a page.
For an online banking app, engagement may be check an account balance.
For a college site, engagement may be watching an informational video.