Calomel n. : Mild chloride of mercury, Hg2Cl2, a heavy, white or yellowish white substance, insoluble and tasteless, much used in medicine as a mercurial and purgative; mercurous chloride. It occurs native as the mineral horn quicksilver.

Example:
why ion selective electrodes were replaced by Calomel electrodes for pH measurements?

Taxonomy
Mercury, Hg, Hg2Cl2, Mercurous chloride, chloride, Calomel, compounds, reaction, Calomel electrode, Cl, free encyclopedia, electrodes, patients, mercuric chloride, elemental mercury, bond length, mineral, History.

Defination from the web:
calomel (a mineral form, rarely found in nature) or mercurous chloride, this dense white or yellowish-white, odorless solid is the ... Calomel was taken internally and used as a laxative and disinfectant, as well as in the treatment of syphilis, until the early 20th century. ...