Trimaximal mixing (also known as threefold maximal mixing) refers to the highly symmetric, maximally CP-violating, fermion mixing configuration, characterised by a unitary matrix () having all its elements equal in modulus (, ) as may be written, e.g.:
where and are the complex cube roots of unity. In the standard PDG convention, trimaximal mixing corresponds to: , and . The Jarlskog -violating parameter takes its extremal value .
Originally proposed as a candidate lepton mixing matrix, and actively studied as such (and even as a candidate quark mixing matrix), trimaximal mixing is now definitively ruled-out as a phenomenologically viable lepton mixing scheme by neutrino oscillation experiments, especially the Chooz reactor experiment, in favour of the no longer tenable (related) tribimaximal mixing scheme.