A woman and/or man who is typically seen at nocturnal events due to physical and/or mental retardation. Commonly used to recover from past relationship and/or rebounds but are never seen during the day with there partner due to the fact respectable people of there community will see them together and discredit the non-nightbitch.
Ted: How come I only see Lisa at your parties?
Bill: Because shes Tommy's nightbitch.
A rude person who obsesses over bands such as Nightwish, who write overly-poetic and overly-dramatic lyrics that depressed kids confuse with being beautiful and deep. They more often than not have some kind of deluded crush (or borderline obsession) on keyboardist/lyricist Tuomas Holopainen and suffer from Münchausen by Internet.
Or anyone you dislike who likes Nightwish
You should have known Melissa's a Nightbitch.