| a pattern of brain waves having a regular frequency of 4 to 7 cycles per second as recorded by an electroencephalograph, observed during various states of light sleep or arousal. |

| theta rhythm n. A waveform on an electroencephalogram having a frequency of 4 to 8 hertz, recorded chiefly in the hippocampus of carnivorous mammals when they are alert or aroused. Also called theta wave. |