The word “nightmare” once referred to what we now called sleep paralysis. The “mare” part of the word “nightmare” did not mean “female horse” but rather originated in the Anglo-Saxon word “merran”, meaning “to crush”. The feeling of being crushed, suffocated, pressed down upon, etc. in sleep paralysis – the original nightmare – is known across the globe in all cultures.
Nowadays, however, the word “nightmare” has a much broader usage, and refers to all dreams that are very emotionally intense (usually fear), which are so intense that they wake the dreamer up. They are different from bad dreams, which do not wake the dreamer up.