What is probability? There are three kinds.

1) Knowledge probabilities. These are numbers that humans (or other intelligent systems) assign to propositions depending on how likely they are to be true. For instance, consider the proposition “The moon is made out of cheese.” If a human knows anything about anything, they would give that a low probability. These probabilities are studied by Bayesian statistics. They are usually just called “probabilities.”

2) Frequency probabilities. If an experiment results in a certain outcome some fraction of the time, that fraction is a frequency probability. Or, if there are N systems and n/N of them are in some state, that is also called a frequency probability. These probabilities are studied by statistical mechanics and frequentist statistics. They are usually just called “frequencies.”

3) Quantum probabilities. In quantum mechanics, a normalized state can be expanded in an orthonormal basis, and then the absolute squares of the coefficients are probabilities. Or to put it another way, the absolute square of the inner product of normalized state A with normalized state B is the quantum probability of A given B.