noun, plural mer·cu·ries.

  1. Chemistry. a heavy, silver-white, highly toxic metallic element, the only one that is liquid at room temperature; quicksilver: used in barometers, thermometers, pesticides, pharmaceutical preparations, reflecting surfaces of mirrors, and dental fillings, in certain switches, lamps, and other electric apparatus, and as a laboratory catalyst. Symbol: Hg; atomic weight: 200.59; atomic number: 80; specific gravity: 13.546 at 20°C; freezing point: −38.9°C; boiling point: 357°C.
  2. Pharmacology. this metal as used in medicine, in the form of various organic and inorganic compounds, usually for skin infections.
  3. (initial capital letter) the ancient Roman god who served as messenger of the gods and was also the god of commerce, thievery, eloquence, and science, identified with the Greek god Hermes.
  4. (initial capital letter) Astronomy. the planet nearest the sun, having a diameter of 3031 miles (4878 km), a mean distance from the sun of 36 million miles (57.9 million km), and a period of revolution of 87.96 days, and having no satellites: the smallest planet in the solar system.
  5. a messenger, especially a carrier of news.
  6. any plant belonging to the genus Mercurialis, of the spurge family, especially the poisonous, weedy M. perennis of Europe.
  7. Good-King-Henry.
  8. (initial capital letter) Aerospace. one of a series of U.S. spacecraft, carrying one astronaut, that achieved the first U.S. suborbital and orbital manned spaceflights.

noun plural -ries

  1. Also called: quicksilver, hydrargyrum a heavy silvery-white toxic liquid metallic element occurring principally in cinnabar: used in thermometers, barometers, mercury-vapour lamps, and dental amalgams. Symbol: Hg; atomic no: 80; atomic wt: 200.59; valency: 1 or 2; relative density: 13.546; melting pt: –38.842°C; boiling pt: 357°C
  2. any plant of the euphorbiaceous genus MercurialisSee dog’s mercury
  3. archaic a messenger or courier


  1. Roman myth the messenger of the godsGreek counterpart: Hermes


  1. the second smallest planet and the nearest to the sun. Mean distance from sun: 57.9 million km; period of revolution around sun: 88 days; period of axial rotation: 59 days; diameter and mass: 38 and 5.4 per cent that of earth respectively

“the Roman god Mercury,” mid-12c., from Latin Mercurius “Mercury,” originally a god of tradesmen and thieves, from merx “merchandise” (see market (n.)); or perhaps [Klein, Tucker] from Etruscan and influenced by merx. Later he was associated with Greek Hermes. The planet closest to the sun so called in classical Latin (late 14c. in English). A hypothetical inhabitant of the planet was a Mercurean (1855) or a Mercurian (1868). For the metallic element, see mercury. n.silver-white fluid metallic element, late 14c., from Medieval Latin mercurius, from Latin Mercurius (see Mercury). Prepared from cinnabar, it was one of the seven metals (bodies terrestrial) known to the ancients, which were coupled in astrology and alchemy with the seven known heavenly bodies. This one probably so associated for its mobility. The others were Sun/gold, Moon/silver, Mars/iron, Saturn/lead, Jupiter/tin, Venus/copper. The Greek name for it was hydrargyros “liquid silver,” which gives the element its symbol, Hg. Cf. quicksilver. n. Symbol Hg

  1. A silvery-white poisonous metallic element, liquid at room temperature, used in thermometers and various pharmaceuticals including antiseptics, diuretics, and antibacterials. Its radioisotope Hg-197 is used in diagnostic imaging of renal function and in brain scans. Atomic number 80.hydrargyrum


  1. A silvery-white, dense, poisonous metallic element that is a liquid at room temperature and is used in thermometers, barometers, batteries, and pesticides. Atomic number 80; atomic weight 200.59; melting point -38.87°C; boiling point 356.58°C; specific gravity 13.546 (at 20°C); valence 1, 2. See Periodic Table.

  1. The planet closest to the Sun and the smallest in the solar system. Mercury is a terrestrial or inner planet, second in density only to Earth, with a rugged, heavily-cratered surface similar in appearance to Earth’s Moon. Its rotational period of 58.6 days is two-thirds of its 88-day orbital period, thus, it makes three full axial rotations every two years. Mercury’s atmosphere is almost nonexistent; this fact, which produces rapid radiational cooling on its dark side, together with its proximity to the Sun, gives it a temperature range greater than any other planet in the solar system, from 466° to -184°C (870° to -300°F). Because it is so close to the Sun, Mercury is only visible shortly before sunrise or after sunset, and observation is further hindered by the fact that its light must pass obliquely through the lower atmosphere where it is distorted or filtered by dust and pollution. See Table at solar system.

The Roman name of Hermes, the messenger of the Greek and Roman gods. In astronomy, the planet closest to the sun, named after the fleet-footed messenger of the Roman gods (see under “Mythology and Folklore”) because of its swift movement in its orbit. Mercury takes only eighty-eight days to go around the sun. (See solar system.) In chemistry, a heavy, silvery metallic element, a liquid at normal temperatures. Mercury expands or contracts rapidly in response to changes in temperature and therefore was once widely used in thermometers.

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