In lower classes viz sixth standard, the first letter of the word "density" ie 'd' is used.But later in higher studies instead 'd', 'rho' the greek letter is used as the symbol for density.
rho (the Greek letter ρ) or d See the Web Links to the left for a picture. The official symbol of density, after ISO recommendations, is the Greek letter ρ (pronounced rho and writed with italic...