Mercury is a planet in the solar system. It comes into conjunction with Mars and Uranus only once every eighty-four years.

On January 12, 1996, a rare planetary alignment involving Mars, Mercury and Uranus occurred. On that date, FBI Special Agent Fox Mulder was investigating a series of murder cases in Comity. Zirinka, a female astrologist, told Mulder that the inhabitants of the small town were acting strangely due to the planetary alignment. (TXF: "Syzygy")

Mercury is also the name of a metal. (MM: "Bardo Thodol")

Mercury vapor lights would sometimes be used at construction sites, but they could restrict a man's ability to adjust his vision to the dark. (TXF: "Fearful Symmetry")

A Mercury Villager was a model of car produced in 1997. (TXF: "Arcadia")

Mercury is referred to in the title of the Season 3 Millennium episode "Saturn Dreaming of Mercury". However, there is no references to Saturn or Mercury in the episode itself.