Einstein's \emph{last} derivation of $E = mc^2$

Einstein's last derivation of $E = mc^2$ is wonderfully simple and compelling. Before sketching a variation on that derivation, I will present some history about the word ``mass" and settle on what the word means in our context. After the derivation, I will address the question, what did Einstein mean in 1907 when he introduced the phrase, ``the equivalence of mass and energy"?