Entrainment is a phenomenon by which some external sensory stimulation synchronizes brainwaves differently than the native rhythm. The most obvious example of this is photic driving – during an EEG the subject will have a strobe light flashed before them at various frequencies. The purpose of this is to see if it will trigger seizure activity. In many normal subjects the brain wave rhythm in the occipital lobes, which is the visual part of the cortex, will match its frequency to the frequency of the strobe light. This specifically is called photic driving, but the phenomenon in general is called entrainment.
If brainwave entrainment leaves you with unwanted side-effects (see below) or discomfort, you’re probably encouraging a range of brainwaves that are already excessive in some area of your brain. The way around this is to get a brain map to see what your brain’s strengths and weaknesses are, and see what (if any) brainwaves could use some encouragement. 
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