 Quantum entanglement is in the first instance a relation between not physical but mathematical objects representing the states of quantum systems. Different forms of quantum theory represent quantum states of various systems by different kinds of mathematical object. So the concept of quantum entanglement has been expressed by a family of definitions, each appropriate to a specific form and application of quantum theory (see Earman (2015)). The first definition (Schrödinger (1935)) was developed in the context of applications of ordinary non-relativistic quantum mechanics to pairs of distinguishable particles that have interacted, such as an electron and proton.