At which 1884 conference was the Greenwich meridian chosen as the prime meridian?

The International Meridian Conference chose the Greenwich meridian as the prime meridian in 1884.

Representatives from 25 nations met in Washington, D.C., from 1 to 22 October 1884 to establish a worldwide reference for longitude and timekeeping. The conference adopted the meridian passing through the centre of the transit instrument at the Royal Observatory in Greenwich, England, as the international zero of longitude.

The decision made Greenwich the starting line for measuring east and west longitude and helped standardize global time zones. The conference also adopted a universal day beginning at midnight and recommended a system based on Greenwich Mean Time. France supported a Paris-based meridian but accepted the vote, and the Greenwich meridian remained internationally dominant until the modern reference system replaced it with a geocentric meridian in 1984.