Differentiation refers not to being "different" in the world at large, but to whether you and your partner can tell where you end and the other begins.
A Schnarshian (referring to Schnarch, the author of Passionate Marriage, Resurrecting Sex) therapist that my bf and I went to gave us these guidelines for being differentiated...
I started to write them out, but they're so important and apply to everyone here, so I think they deserve their own thread.