From WordNet (r) 3.0 (2006) [wn]:
midnight
n 1: 12 o'clock at night; the middle of the night; "young
children should not be allowed to stay up until midnight"
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Midnight \Mid"night`\, n. [AS. midniht.]
The middle of the night; twelve o'clock at night.
[1913 Webster]
The iron tongue of midnight hath told twelve. --Shak.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Midnight \Mid"night`\, a.
Being in, or characteristic of, the middle of the night; as,
midnight studies; midnight gloom. "Midnight shout and
revelry." --Milton.
[1913 Webster]