I first learned about "differentiation" reading David Schnarch's Passionate Marriage a couple of decades ago. The word itself came from the field of mathematics in the 1600s--think Isaac Newton. Over time, its meaning expanded into the fields of geology and biology. Schnarch's psycho-social perspective repurposed the word to describe the progressive, internal interplay of humans between autonomy (separation) and connection (togetherness). Schnarch is a sex therapist. Differentiation, however, goes far deeper as a practice than a remedy for a couple's sexual gridlock.
Differentiation is the ability to hang on to your unique self and what you need, even when those around you, including those you love, disagree. Differentiation is navigating your own boat and allowing others around you to navigate theirs. As long as we sail down life's river maintaining status in our own vessel, the trip can be smooth. If we find ourselves climbing onto the boat of another to adjust their navigation (control how they feel or travel the journey, or blame them for how we feel), we create a hazard. it's time to step back, climb back on our own deck and reflect on why we feel the need to navigate for others. There's plenty of work to be done on our own boat. In the end, differentiation is a gentle push for us to be all we can be by being who we are, and occasionally growing up and out of obsolete ways of being, especially in our relationships.