Beta wave was a term in neurology for describing the electrical activity in the brain of an individual when they were mentally active.
In 3189, Doctor Hugh Culber measured the beta waves of the USS Discovery crew. Then, the results were compared to a baseline average in the graphic "Crew Health", which was used as a visual aid in the physician's report on the state of the crew's health to Captain Saru. (DIS: "Forget Me Not")
Later that year, an atomic scan was performed on the brain of Philippa Georgiou. The results of the scan revealed that her beta wave was normal with a frequency range of 17-29 Hz. (DIS: "The Sanctuary")