  We propose an order parameter for a general one-dimensional gapped system
with an open boundary condition. The order parameter can be computed from the
ground state entanglement entropy of some regions near one of the boundaries.
Hence, it is well-defined even in the presence of arbitrary interaction and
disorder. We also show that it is invariant under a finite-depth local quantum
circuit, suggesting its stability against an arbitrary local perturbation that
does not close the energy gap. Further, it can unambiguously distinguish
Majorana chain from a trivial chain under a global fermion parity conservation.
We argue that the order parameter can be in principle measured in an optical
lattice system.
