This is shorthand which the authors Jeff Miller & Pam Hoffman use which stands for 'Conservation of Eggs' the overriding name of the series of books; the first of which is titled "Where's My Other Half?"
Have You Ever Been Stuck, With One Egg Yolk or One Egg White After Using a Recipe?
The first book has simple recipes that provide you with ways to use the 'other half' of the egg.
We encourage you to "Stop Wasting Yolks!"
You may see CoE in various places here on the blog, in the books, on our list, etc.