a surface of constant density
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 2 words with the pattern ---P-C--L. That is, nine letter words with 4th letter P and 6th letter C and 9th letter L. In alphabetical order, they are: isopycnal mispickel
Atmospheric and oceanographic sciences generally differentiate between vertical convection and horizontal advection. These disciplines study the atmosphere and the oceans which are both much wider in the horizontal than they are thick in the vertical. Making the distinction between horizontal (i.e. isopycnal, along surfaces of constant density) and vertical (i.e. diapycnal, through density surfaces) therefore makes sense for meteorologists and oceanographers. In fluid mechanics on the other hand the terminology is often used interchangeably. Their study environment is frequently a tank in a laboratory and therefore is more of a cube compared to the thin oceanic layer on the Earth. In fluid mechanics, advection and convection is the same.