someone extremely diligent or skillful
"he worked like a demon to finish the job on time"
"she's a demon at math"
An evil supernatural spirit.
"The ancient legend warned that a demon would rise from the swamp to torment the villagers at midnight."
An evil spirit resident in or working for Hell; a devil.
"The terrified villagers prayed to their saints as they saw a demon rising from the flames of Hell to claim another soul."
In plain English: A demon is an evil spirit that people believe can cause harm or trouble to humans.
"The movie features an evil demon who causes trouble in hell."
Usage: Use demon to refer specifically to an evil supernatural being from folklore, religion, or mythology rather than using it as a metaphor for difficult people. Avoid confusing the mythical creature with abstract concepts of bad luck or general misfortune which require different vocabulary.
A male given name from Ancient Greek.
"Demon is an unusual but growing first name chosen by parents seeking a unique origin from ancient Greece."
The word "demon" comes from the Medieval Latin dēmōn, which originally meant a familiar or guardian spirit borrowed from Ancient Greek. While its root referred to a divine protector rather than an evil entity, the term traveled into English with this ancient meaning before evolving over time.