  Under the assumption that superconducting pairing is driven by local d-p
hybridization, we show that the superconducting state in iron-based
superconductors is classified as an odd parity s-wave spin-singlet pairing
state in a single trilayer FeAs/Se, the building block of the materials. In a
low energy effective model with only d-orbitals in an iron square bipartite
lattice, the superconducting order parameter in this state is a combination of
a s-wave normal pairing between two sublattices and a s-wave $\eta$-pairing
within the sublattices. Parity conservation was violated in proposed
superconducting states in the past. The results demonstrate iron-based
superconductors being a new quantum state of matter and suggest that a
measurement of odd parity can establish fundamental principles related to high
temperature superconducting mechanism.
