The dark side of dreams
In common terms, nightmares are equivalent to bad, frightening dreams. Where does their name come from? In old times people used to think nightmares were caused by a species of demons called 'incubi', which were thought to sit on the chests of sleepers. In Old English the name for these beings was 'mare', hence comes the 'mare' part in 'nightmare'.
The modern explanation, of course, has nothing to do with the naughty incubi. Nightmares are a sign that somewhere, deep in our subconscious mind, there's something that shouldn't be there. People who experience recurring nightmares usually have one or more unsolved problems in their life. It might be a phobia, a fear buried deep inside, a regret or remorse or an earlier trauma. Their subconscious tries to warn them of those problems by sending them a repeated message. After the problem that causes them is solved, they usually stop.
Nightmares are very vivid and they provoke panic and terror. The tension increases gradually until the dreamer finally wakes up. They can cause physical reactions such as sweating, acceleration of the heart rhythm, the sensation of choking, etc.
In order to treat the simptom, one needs to cure the disease, in order to get rid of the unwanted effects, one needs first to identify the cause. So don't let your nightmares scare you. Rather than regarding them like enemies, treat them as if they were messengers from your subconscious. Don't try to ignore them, but look for the cause within yourself and try to eliminate it.