Definitions of mercury:

• noun:   temperature measured by a mercury thermometer

  Example: "The mercury was falling rapidly"

• noun:   the smallest planet and the nearest to the sun
• noun:   (Roman mythology) messenger of Jupiter and god of commerce; counterpart of Greek Hermes
• noun:   a heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures
• name:  A surname (very rare: popularity rank in the U.S.: #55488)