There is no consistent quantifiable definition of engagement across different products, but if you make user engagement a priority there are loads of quick wins to be had.

Intercom on Product Management

Chapter 8: How to increase user engagement

How do you build something people actually want and will use time and time again? Sharing lessons we've learned in the six-plus years building Intercom, in this series you'll learn how to evaluate your current product, how to decide which features to build in a haystack of those you shouldn't, best practices for getting those features used, and much more.

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Intercom on Product Management