Subject: I understand why we don't use 'd' for density, but why rho?

Well, this question appeared in an applied physics class over the summer. The
greek letter rho is used for all kinds of volumetric densities--charge, mass, et
cetera--but nobody seems to have a good reason why. The best I can find is that
"d means derivative." Is there a more in-depth reason? Does it stand for a Greek
word? Or an old meaning for the word?

Re: I understand why we don't use 'd' for density, but why rho?