In answer to someone's question, it refers to this:http://passionatemarriage.com/passionatemarriage/about-book
"...a passionate sex life requires each person to face the anxiety of defining himself/herself while getting closer to their partner, a process [David Schnarch] calls differentiation.
Differentiation involves changing the way we think about...[relationships]: Instead of seeing ...[them] as the merging of two people into one, as has often been taught, we must learn to maintain a sense of ourselves as distinct from our partner[s] in order to become closer to [them]. Gaining more differentiation is not easy-and [David Schnarch] warns that any "expert" who promises Eros and intimacy in ten easy steps should not be trusted. Sexual encounters provide perfect opportunities to differentiate and develop the strength to love deeply."
I got this concept from working directly with David Schnarch a handful of years ago and it has served me incredibly well. Although I learned about it in the context of relationship therapy and sexual intimacy, I could immediately see how it applied just as well to non-sexual relationships like those we have with our parents, mentors, long-term teachers and close friends.
If you want to learn more about differentiation, I highly recommend that you read his book, "Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships"
In fact, I think it's required reading for 3rd Degrees and even 2nd Degrees, because the concept helps explains so much of the conflict that arises between students and teachers in long-term Craft relationships.
And for anyone obsessed with maintaining agency in intimate relationships, it's a must-read.