with regard to electroencephalography, a form of brain wave with a frequency of 4-7 hertz. Theta waves are viewed in animal's REM sleep, phase two sleep in human beings, and within the drowsiness state of newborn babies, teens, and young adults. Theta waves are also documented in hypnosis, deep daydreams, and trances. Neurologically, the hippocampus is one highly recognized source of theta activity.
THETA WAVE: "Theta waves are commonly referred to as theta rhythms."