A cauterizer is a technological device, a medical instrument used to burn tissue in order to seal a lifeform's wound from microbes and other pathogens. Aboard Federation starships in the 23rd century, this medical device was available among the tools and instruments used by the Starfleet chief medical officer, doctors and medics.
In the year 2268, James T. Kirk noted Odona putting her hand on a cauterizer in a facsimile of Doctor McCoy's work area in sickbay aboard a replica of the USS Enterprise. He tried to warn her of the danger, but before he could stop her, she activated the device and completely burned off one of her fingers. Kirk was shocked at her inattention to what looked like a painful injury, but within moments her finger had completely regrown due to her Gideonite physiology. (TOS - Star Trek 6 novelization: The Mark of Gideon)