What is Employee Engagement?

In the HBR.   Definitions, expectations of results?   " ... This holistic approach to understanding engagement will yield more-detailed insights into what makes people stick around and do their best work. Instead of viewing engagement in terms of low, medium, and high, organizations will be able to understand how employees perceive them, how that perception relates to their behavior, and in aggregate, how those factors drive bottom-line performance. If organizations don’t dig deep like this, they risk misunderstanding their employees and missing out on all the benefits of high engagement. ...  "