Fluid or liquid? Perfect fluid? A perfect gas obeys the pv = RT state equation. I suppose that means the specific heat capacity is well behaved over a range of temperatures, which determines what R is for that gas.
'Ya got me, RW. None of the other laws of physics are known to hold inside a black hole, so I'm not sure this one would hold, either. How do you define pressure when the mass of a billion stars has been squeezed down to a mathematical point and the volume is zero? Very, very little of this stuff makes any sense to me. But I love it nonetheless! :-)