Video footage with a filter showing body temperature
Body temperature was the thermal reading of an individual's body. It could be measured in degrees Fahrenheit or Celsius. The body function panel in USS Enterprise's sickbay included a temperature reading indicator with both of these measurement scales. (Star Trek: The Original Series)
Normal body temperature was one of the markers that confirmed an individual's species. In 2267, Lazarus's body temperature registered at 98.5 degrees Fahrenheit. (TOS: "The Alternative Factor") In 2268, Arne Darvin's body temperature helped to convince Doctor McCoy that he was in fact a Klingon. (TOS: "The Trouble with Tribbles")
A sudden change in one's body temperature could be an indicator of illness or impending death. In 2268, Hodin told Odona that a sudden increase in body temperature would precede her death. (TOS: "The Mark of Gideon") In 2367, Susanna Leijten's body temperature dropped as she changed form. (TNG: "Identity Crisis")