Dream vs Nightmare

What is a Dream?

In contrast to nightmares, which are just dreams that elicit a strong but unpleasant emotional reaction, dreams are generally thought to be recent autobiographical incidents that become interwoven with earlier memories to generate a new memory that can be referred to later. Dreams are a function of the default network, a system of interconnected areas in the brain that includes the thalamus, medial prefrontal cortex, and posterior cingulate cortex and are active even when the rest of the brain is relatively quiet.

What is a Nightmare?

A nightmare is a frightening dream that causes you to wake up and is connected to unpleasant emotions like fear or worry. Children frequently experience nightmares, but anyone can experience them. Typically, occasional dreams are nothing to be concerned about.
Though prevalent, nightmare disorder is somewhat uncommon. The term "nightmare disorder" refers to a condition in which dreams frequently occur, cause anxiety, interfere with sleep, impair everyday functioning, or instill a fear of falling asleep.

References

<https://hms.harvard.edu/news-events/publications-archive/brain/nightmares-brain#:~:text=Dreams%20are%20understood%20to%20be,strong%20but%20unpleasant%20emotional%20response.>.

<https://www.mayoclinic.org/diseases-conditions/nightmare-disorder/symptoms-causes/syc-20353515>