Why Pauli exclusion principle and not Pauli exclusion law?

Question

Why is the Pauli exclusion principle called a principle and not a law?

Answer 1

There's a myth among those outside of academia that scientists have perfectly circumscribed definitions for things like principle, law, hypothesis, theory, etc. The truth is that aesthetics plays the largest role in how things are named.

Answer 2

Although there is no standardization in the language used in Physics, the established usage of the words makes a difference between law and principle. The former is equivalent to a statement describing the behavior of a physical system under controlled circumstances. The latter is a law that is deemed to be foundational for a sector of Physics. A law may also correspond to an implicit definition of a physical concept when used as a principle.

Consequently, a statement about the behavior of Nature (a law) may be considered a principle in some formulation of the theory and not in another.

The so-called Pauli principle originates from old Quantum Mechanics, where it was considered foundational for the theory of atomic spectra. Nowadays, we know that its original formulation in terms of quantum numbers describing one-particle states is not general enough, and the antisymmetry of the fermionic wavefunction can be considered a principle in some formulations of quantum mechanics (although it becomes a theorem, the spin-statistics theorem, in QFT).