The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time throughout most of the world. Also called Greenwich time, Greenwich Mean Time, Zulu time. Mean (solar) time of the Greenwich meridian (0° longitude). Universal Time replaced the designation Greenwich Mean Time in 1928; since 1972 it has been based on international atomic time, a uniform time derived from the frequences of certain atomic transitions and measured by an atomic clock. For most purposes, it is now identical to Greenwich Mean Time