K3 Indicator
From Star Trek : Freedom's Wiki
Medical measurement of neural activity corresponding to the level of pain bieng experienced by a patient.
While treating Spock for neural parasite infiltration, Doctor McCoy activated this indicator to illustrate to Captain Kirk how much pain the parasite was inflicting. The indicator rose to about 75% of its maximum; McCoy commented that it was "no wonder the poor devils go mad"; implying that this was a considerable degree of misery. (TOS: "Operation -- Annihilate!")
