In quantum information and quantum computation, an entanglement monotone or entanglement measure is a function that quantifies the amount of entanglement present in a quantum state.
Any entanglement monotone is a nonnegative function whose value does not increase under local operations and classical communication.
be the space of all states, i.e., Hermitian positive semi-definite operators with trace one, over the bipartite Hilbert space
An entanglement measure is a function
over the maximally entangled state
If the nonnegative function only satisfies condition 2 of the above, then it is called an entanglement monotone.