No, entanglement does not just mean “it is in one of these two states but we haven’t measured which”. Entanglement is what you get when you have a state in the

No, entanglement does not just mean “it is in one of these two states but we haven’t measured which”. Entanglement is what you get when you have a state in the tensor product of two systems which is not a tensor product of a single state from each system, but is instead a linear combination of such products. There are, in fact, experiments that can be (and have been) done which distinguish between “these two particles are entangled” and “we don’t know which of two combinations of states these two particles are in”. However, if you only have one particle of the entangled pair, and don’t have any other way of getting information from the other particle, you can’t distinguish it from just, a probability mixture of (superpositions of) states of the one particle. I say “(superpositions of) states of the one particle” to emphasize that the states don’t have to be in one of the standard basis states with a nice name like “up” or “down”, (though you could use a different basis in which it is a basis state).