rhythm-, rhythmo- +

(Greek: regularly recurring motion; measured motion)

1. Tachycardia; an excessively or abnormally rapid heart rate.
2. Increase in the frequency of brain waves in electroencephalography up to 12 to 50 per second.
theta rhythm, theta wave
1. 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.

It occurs naturally in children up to about 12 years of age, but it is considered abnormal in adults.

2. The normal brainwave in the encephalogram of a person who is awake but relaxed and drowsy.

It occurs with low frequency and low amplitude.

3. A relatively high amplitude brain wave pattern between approximately 4 and 9 hertz that is characteristic; especially, of the hippocampus but occurs in many regions of the brain including the cortex.
unrhythmic, unrhythmical
Not rhythmic; that is, irregular in beat or accent.