Which city is the reference point for Greenwich Mean Time?

Greenwich is the city-area reference point associated with Greenwich Mean Time.

GMT is historically the local mean solar time at the Royal Observatory in Greenwich, now part of London. The observatory's location supplied the name and became the reference for measuring longitude and expressing time differences around the world. The Prime Meridian was traditionally identified with the Greenwich meridian at 0 degrees longitude.

GMT became especially influential during the nineteenth century, when railways needed a consistent timetable. British railway companies adopted Greenwich time in the 1840s, and it was legally adopted throughout Great Britain in 1880. The famous time ball at the observatory and later radio signals helped distribute the standard.

GMT and UTC are often treated as synonyms in everyday speech, but they are not identical concepts. GMT is based on mean solar time, while UTC is an atomic-clock standard adjusted to stay close to Earth's rotation. UTC replaced GMT as the international civil-time standard in 1972. Lisbon, Berlin, and Paris may use time offsets related to UTC, but Greenwich is the canonical reference named in the question.