These are brain waves in the frequency band 1 - 3 Hz.
The delta waves are high and wide.
They normally occur during deep dreamless sleep and in infants before the age of 1.
Delta waves occur in connection with unconsciousness as well as when one is in a trance state; type hypnosis, deep meditation.
The delta waves also appear to occur when, for some reason, one loses concentration during the performance of a task.
Delta waves occur in individuals with attention deficit hyperactivity disorder (ADHD), especially in the variant where attention deficit hyperactivity disorder predominates. When such a person is faced with a specific task, it has been observed that delta waves begin to occur when the person's commitment to the task decreases and then ceases.
Delta waves are sometimes claimed to be a sign that the brain is disconnecting from physical reality and finding its way down to its subconscious wards.