What does R. C. Sproul mean by the 'essence' of God?

[R. C. Sproul](https://en.wikipedia.org/wiki/R._C._Sproul) discusses and defends the Trinity in this book, [*What is the Trinity?*](http://www.ligonier.org/blog/what-trinity/) He explains that God is one *essence*, composed of three *persons*, and that the existence of three persons in one essence does not contradict the law of non-contradiction. Now, what does he mean by 'essence'? Why does he use the term 'essence' instead of 'entity'? Does he use the term 'essence' in the same way he would use it in 'the essence of a squirrel' or 'all the squirrelness about the squirrel that makes up the squirrel'? I realize that there is a similar question: https://christianity.stackexchange.com/questions/15659/what-do-we-mean-by-person But this one is more focused on the word, *essence*, which I believe is a technical term in Christianity, not the word *person*.