Entrainment is defined as the action of adjusting the phase or period of a waveform by introducing it to another waveform.
The term, 'Entrainment', was created by, Christiann Huygens, a Dutch mathematician, physicist and inventor of the pendulum clock . He noticed that when he hung a new clock on the wall, its pendulum would settle towards moving in sync with the other clocks. Huygens observed that each new clock would always harmonize (i.e.: 'entrain') to the back and forth frequency of the other pendulums, swinging in unison on the wall.
It is revealed by circadian biologists that organisms from plants to mammals have evolved a marvelously complex array of responses which guarantee the precise alignment of biological processes to environmental time and frequency.
One reliant entrainment phenomenon is your Circadian rhythm, or your 'internal clock'. Your body's internal processes are dependent on your entrainment to the frequency of 'day and night'. In fact, the brain has a special area (the suprachiasmatic nucleus) which regulates your body's cycles by the entrainment of light and darkness.
Research has proven that introducing a binaural beat will cause the brain to begin resonating in tune with the frequency of that beat. This is an effect of the called the brain's Frequency Following Response and was thoroughly researched and tested in by biophysicist Gerald Oster in 1973. Oster's research on binaural beats, and the frequency following response, paved the way for further development in auditory stimulation when his work was later published in Scientific American.
...today these developments continue.