A white or grayish powder made from mercury and chlorine, historically used in medicine as a laxative and disinfectant.
From Greek 'kalos' (beautiful) and 'melas' (black), named by alchemists because heating it turns it black, though the powder itself appears white. The term was coined in the 17th century when chemists were naming mercury compounds based on their appearance changes.
Calomel is a perfect example of how dangerous 'medicine' used to be—doctors prescribed it for everything from constipation to teething pain in babies, not realizing they were slowly poisoning patients with mercury. It's one of the reasons the history of medicine includes some truly wild and harmful treatments.
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