n. a (fictional) place where demons/other vile stuff can come up to the surface of Earth. Sunnydale is on top of a Hellmouth, so is Cleveland (in the series.)

I've been to Cleveland...has to be a Hellmouth, lol

When you get up in the morning after eating loads of garlic the night before:-)

It's a place where there's like a door between Hell and earth. So there is a sort of hellish energy around the place that attracts evil beings and monsters etc. If the hellmouth were opened, then all sorts of creatures would come out of hell and rampage around earth.

Hellmouths are places of increased supernatural energy. According to the mythology of the "Buffyverse", this is the area in which the barriers between dimensions are weak. The Hellmouth has a focal point, which serves as a portal between earth and Hell. For these reasons, the Hellmouth attracts demons and other supernatural creatures, becoming a "hot spot" for supernatural activity. The television series has confirmed that there are Hellmouths under Sunnydale, California and Cleveland, Ohio, as well as other underspecified cities around the world (cf. "Chosen"). The Hellmouth is first mentioned by name in "The Harvest", the second episode of Buffy, when Rupert Giles claims that the original Spanish settlers of Sunnydale called the area "Boca del Infierno," which Giles then translates to "Hellmouth" in English. The energies emitted by the Hellmouth can have strange effects. Sunnydale High, positioned over the focal point, bore most of this supernatural phenomena. Buffy described the effects thus; "where the way a thing feels kind of starts being that way for real". For example, a shy girl who feels invisible, becomes invisible and is taken away to a school for other invisible children. A boy suffering from stress literally explodes. The ghosts of dead high-school lovers haunt the school. In Season 7, when the Hellmouth becomes more active than during any other time in the series, competition between social cliques escalates into full-blown war. Inexplicable feats of science become possible. People (mostly teachers and students as well as police) become drawn to do evil things. In both "Prophecy Girl" and "The Zeppo", large, multi-tentacled demons seem to be immediately inside the Hellmouth, until Buffy Summers and her friends slew them in the latter episode. In the episode "Doomed", Buffy entered the Hellmouth where it was revealed to be a large, empty cavern. Three years later, the First Evil bred thousands of Turok-Hans inside the Hellmouth, preparing to unleash a worldwide assault.