[Kory]

I'm no expert,
But to me, any dream that perturbs or disturbs you would count as a nightmare.

I tend to have really bad dreams about the lights not working. So like, sometimes I'll be having a very basic dream, but then I go to turn the lights on and they don't work...

I'm extremely afraid of the dark, so immediately I start to panic and freak out when the lights don't work in my dreams. ;-;

It might just be called a "bad dream" instead of full on nightmare.
But still sounds scary!