means The middle of the night, twelve o'clock, the precise moment when one calendar day ends and the next begins.

from A plain old compound from Old English 'midniht' — literally 'mid' (middle) plus 'niht' (night). Both halves trace back through the Germanic family: 'mid' is a cousin of Latin 'medius' and Greek 'mesos' (middle), while 'night' shares deep roots with Latin 'nox' and Greek 'nyx'. So the word is exactly what it says — the night's midpoint — and it has stayed that honest for over a thousand years.