Interactions — likes, comments, shares, saves — divided by reach or followers, depending on which platform is doing the arithmetic.
Engagement rate is the most quoted and least commercially useful number in social media. It is easy to inflate with giveaways and questions, and it rarely correlates with sales for a local service business.
It has one real use: as a relative measure of what your own audience responds to. Comparing your posts against each other is informative; comparing your rate against an industry benchmark is not.
This comes up most often in Social Media Management. A posting rhythm that looks like your business — planned a month ahead, approved by you, published on schedule.
A free consult is twenty minutes and there is no pitch deck at the end of it.