It’s always been just KDE to me. In the past year or so, I’ve noticed people calling Plasma KDE or even just Plasma. What happened? Why do people insist on making the name longer? Why the word Plasma? What is Plasma referring to? And, are they trying to move away from the KDE name to just Plasma?
Answer: Plasma is the desktop environment. KDE is the community that makes Plasma and many other related software.
Plasma is currently the name of the Desktop Environment made by KDE.
KDE refers to the umbrella project that houses all the software made for Plasma, like KDE Connect and Kdenlive. The name used to refer to the desktop environment; but now they develop more stuff than the desktop environment.