Isopycnic
![Viruses purified by isopycnic centrifugation— a density gradient was formed during high speed centrifugation of a solution of caesium chloride and the two virus types come to rest at points corresponding to their density. The tube is about 10cm long.](/Images/godic/202501/21/Purified_Viruses1815.jpg")
Isopycnic means "of the same density." In particular, an isopycnic surface is a surface of constant density. This term is a bit more obscure than the similar terms isobaric or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively. It is common in conversational use to hear isopycnic surfaces referred to simply as "iso-density" surfaces, which while strictly incorrect, is nonetheless abundantly more clear.