Nightmares can be defined as scary dreams.

1 out of every 4 children has nightmares more than once a week. Most nightmares happen very late in the sleep period (usually between 4 a.m. and 6 a.m.). Your child may wake up and come to you for comfort. Usually, he or she will be able to tell you what happened in the dream and why it was scary. Your child may have trouble going back to sleep. Your child might have the same dream again some other night.
Some children have a different kind of scary dream called a “night terror” which is different from a nightmare. A night terror, also known as pavor nocturnus, is a parasomnia sleep disorder characterized by extreme terror and a temporary inability to regain full consciousness. Night terrors happen during deep sleep (usually between 1 a.m. and 3 a.m.). A child having a night terror will often wake up screaming. He or she may be sweating and breathing fast. Your child’s pupils (the black center of the eye) may look larger than normal. At this point, your child may still be asleep, with open eyes. He or she will be confused and might not answer when you ask what’s wrong. Your child may be difficult to wake. When your child wakes, he or she usually won’t remember what happened. Children from age two to six are most prone to night terrors, and they affect about fifteen percent of all children.
Nightmares and night terrors don’t happen as much as children get older. Often, nightmares and night terrors stop completely when your child is a teenager. Some people, especially people who have active imaginations, may keep having nightmares when they are adults. Nightmares and night terrors in children are usually not caused by mental or physical illness. Often nightmares happen after a stressful physical or emotional event. In the first 6 months after the event, a child might have nightmares while he or she gets used to what happened in the event. If nightmares keep happening and disturb your child’s sleep, they can affect your child’s ability to function during the day, causing insomnia and physical weakness as well. Talk with your doctor about whether treatment will help your child.
Night terrors and sleepwalking require that you protect your child during sleep. Be sure your home is safe (use toddler gates on staircases and don’t use bunk beds for children who have nightmares or night terrors often). Talk with your doctor if your child ever gets hurt while sleeping. Your doctor may want to study your child during sleep. The doctor will be of great psychological help in this case.