The property of matter that two identical particles with half integer spins (fermions) cannot occupy the same quantum state.
https://en.wikipedia.org/wiki/Pauli_exclusion_principle
“In a poly-electron atom it is impossible for any two electrons to have the same two values of all four of their quantum numbers, which are: n, the principal quantum number; , the azimuthal quantum number; , the magnetic quantum number; and ms, the spin quantum number. For example, if two electrons reside in the same orbital, then their values of n, , and are equal. In that case, the two values of (spin) pair must be different. Since the only two possible values for the spin projection are +1/2 and -1/2, it follows that one electron must have = +1/2 and one = -1/2.” This means that in systems of very dense matter, the system {cannot reach homogeneity}, and thus will actually have an inherent pressure. This pressure can prevent collapse due to gravity in various high dense objects (compact objects in astronomy).