There are several synonyms for the term "extraterrestrial vessel". One of the most commonly used terms is "UFO" or "Unidentified Flying Object". This term is used to refer to any object that is seen in the sky that cannot be identified as a terrestrial object. Another synonym is "alien spacecraft". This term is used to refer to any vessel that is believed to be of extraterrestrial origin. Other synonyms for "extraterrestrial vessel" include "flying saucer", "alien ship", "interstellar craft", and "alien spacecraft". These terms are commonly used in science fiction and UFO stories, and are often associated with sightings and encounters with extraterrestrial life.