Nautical dawn is the time at which the sun is 12 degrees below the horizon in the morning. Nautical dawn is defined as that time at which there is just enough sunlight for objects to be distiguishable.
Astronomical dawn is the time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the sun starts lightening the sky. Prior to this time, the sky is completely dark
Dawn should not be confused with sunrise, which is the moment when the leading edge of the Sun itself appears above the horizon.
See also dusk.
In Western tradition it is believed that evil spirits, demons, and even Satan are obliged to disappear at dawn, for being they creatures of Darkness hate light, especially that of the Sun. Pre-Christian Celts also shared this belief.