The knowledge an individual has of future events that are typically tragic in nature. They commonly come as visions, often during dreams and even during states of total consciousness. A strong sense that something will occur also qualifies. There is strong historical evidence of premonitions revealing visions and warnings of future happenings.
Sam is woken from a dream, believing to have had a premonition of a man being suffocated by his car's exhaust fumes. He tells Dean it felt the same as when he dreamt of their old house and of Jessica, which prompts Dean to question why he would be dreaming of a random man in Michigan.
Sam: Dean, it's another premonition. I know it. This is gonna happen, and Ash can tell us where.
Sam: Every premonition I've had, if they're not about the demon they're about the other kids the demon visited. Like Max Miller, remember him?
Andy: So you get these premonitions of people about to die?
Sam: Yeah. Psychic ability. Me, I have, um, I have visions. Premonitions. I don't know, it's, it's different for everybody.
Lucifer: You know, I don't suppose you'd just say yes here and now? End this whole tiresome discussion? That's crazy, right?
Sam: It's never gonna happen!
Lucifer: Oh, I don't know, Sam. I think it will. I think it'll happen soon. Within six months. And I think it'll happen in Detroit.
Lucifer's premonition does come true as Sam says yes in Detroit in the episode Swan Song.