What do the letters in the GMT time zone stand for?

Greenwich Mean Time is correct because GMT is the mean (average) solar time measured at the Royal Observatory in Greenwich, London, located on the Prime Meridian (0° longitude). “Mean” refers to averaging the slight variations in the solar day over the year, creating a stable reference time. Adopted globally at the 1884 International Meridian Conference, GMT became the world’s timekeeping baseline and later paved the way for Coordinated Universal Time (UTC). Today, GMT is still used colloquially and matches UK time in winter. Memory tip: G = Greenwich (Prime Meridian), M = Mean (averaged solar time), T = Time. Ignore distractors like “Glasgow Man Time” or “Global Meridian Time”—only Greenwich anchors the zero-longitude line.