Geo, I don’t know if there is an official answer to that or not. I sometimes refer to any synthetic material as composite, since it consists of more than one material and that is after all the definition of a composite. I think in the knife world it might get a little confusing since Case calls a certain handle material “yellow composite” and “black composite”. For clarity’s sake it might be best to refer to Delrin as Delrin, but when not sure, I say composite or synthetic. That’s just the way I do it, which might not jib with how others think.
“As iron sharpens iron, so one man sharpens another.” Proverbs