A nightmare is a dream that makes one feel fear, by making him/her feel in an uncomfortable or uneasy situation. Nightmares can include oneself dying, being chased, being hurt, or experiencing some otherwise scaring situation. Other examples can be to find yourself lost without being able to get out or spiritual references such as seeing a demon or a spirit of some sort.
Nightmares usually happen because of situations dealt with in real life, such as excessive anxiety or fear, stress and other factors.
Recurring nightmares may also mean the necessity for psychological support, since some trauma or bad situation lived would be provoking the nightmares.
Spiritual dream interpretation almost always sees nightmares as bad omens. The content of a nightmare is considered to be indicading a bad situation for the future.