Either of a pair of elementary particles (W⁺ or W⁻), classified as intermediate vector bosons, that are believed to transmit the weak interaction (see fundamental interactions) in much the same way as photons transmit the electromagnetic interaction. They are not, however, massless like photons, and are believed to have a rest mass of the order of 10⁻²⁵ kg (80.4 GeV). W bosons were discovered at CERN in 1983 with the expected mass. See also Z boson.