The matter which density is not dependent on pressure $\begin{array}{l}p\end{array}$ :

(1)	$\begin{array}{l}\displaystyle \rho(p, T) = \rho_0(T) = {\rm const}\end{array}$

which is equivalent to zero compressibility:

(2)	$\begin{array}{l}\displaystyle c(T, p) \equiv 0 \Leftrightarrow \frac{d \rho}{ dp} = 0\end{array}$

Incompressible matter does not exist but in many practical applications the pressure and temperature variations stay within the sufficiently narrow range where a given matter can be considered as incompressible.

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