Mercury

(noun) a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures. Synonyms: mercury, quicksilver, hydrargyrum, Hg, atomic number 80.

(noun) (Roman mythology) messenger of Jupiter and god of commerce; counterpart of Greek Hermes.

(noun) the smallest planet and the nearest to the sun.

(noun) temperature measured by a mercury thermometer. Synonyms: mercury. Example: "The mercury was falling rapidly."