Universal time is a term that refers to the standard time used by astronomers worldwide. It is based on the rotation of the Earth and is calculated using atomic clocks. Some synonyms for universal time include Greenwich Mean Time (GMT), Coordinated Universal Time (UTC), and Zulu Time (Z). GMT is the mean solar time at the Royal Observatory in Greenwich, London, while UTC is a time standard that is used as the primary time standard by countries around the world. Zulu Time is the military term for UTC and is used in aviation, shipping, and other industries. These terms all refer to a precise, globally recognized standard for measuring time.