A cow-orker recently "informed" me that the reason we call the speed of light $c$ is that c stands for "causality.
I would guess that can't be the case, because I think the symbol $c$ was used to represent the speed of light as early as the days of Faraday and certainly in Maxwell's papers, while the idea that light propagation and causality are related came into being decades later, with relativity.
But I'm not absolutely certain that Maxwell used the symbol $c$.
At any rate, does anyone know when using the symbol $c$ to represent the speed of light came into common practice?