It is something that is assumed at the "beginning" of a theory. The remainder of the theory's predictions are conclusions derived from its postulates. Postulates can be tested for correctness, but they are assumed, not derived.
Some examples of postulates:
The speed of light is constant for all observers.
Gravitational and inertial mass are the same.
Each physical theory should be endowed with an axiomatical structure.From my reading and understanding experience,the best of them all is the one of (Nonrelativistic) Quantum Mechanics.Each if its 6 axioms has more fomulations (wording and mathematical expressions) depending upon the formulation of the theory:Dirac's (a.k.a.traditional),von Neumann's,Feynman's,...
There's one postulate (the IV-th,i.e.the time evolution postulate) which,in every formulation aforementioned,can be expressed in 3 different ways,depending upon the picture one adopts:Schrödinger,Heisenberg or interaction (a.k.a.Dirac-Tomonaga-Schwinger).And then of course you have the representationsccupation number,occupation number-energy,position,momentum.Then of course you have the original formalism of matrix mechanics and wave mechanics,but this is not really a part of the axiomatical structure,as one finds them as particular realizations of the various ways of describing the concept of "physical state".
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