Infernal means related to hell or suggesting the torments of hell. When something is infernal, it's either literally connected to the underworld of fire and demons, or it feels so awful, irritating, or chaotic that it reminds you of what hell might be like.

In stories and myths, infernal creatures are demons or monsters from the underworld. Dante's famous poem describes the infernal regions where sinners face punishment. When fantasy novels describe infernal magic, they mean dark powers connected to evil supernatural forces.

More commonly, people use infernal to describe something unbearably annoying or frustrating. A broken photocopier that jams constantly might be called an infernal machine. A ridiculously difficult puzzle could be described as having infernal difficulty. When your little brother won't stop making that infernal noise with his recorder, you mean it's driving you absolutely crazy.

The related noun is inferno, which means a raging fire or hell itself: a house fire might become an inferno.