The English term "Demon" typically denotes harmful or malevolent spiritual beings found in Judeo-Christian religious traditions. The term can also be used to describe the malevolent types of Jinn from Islamic traditions, or more broadly any harmful, malicious or frightening spiritual beings from non-Abrahamic traditions the world over.
The modern term "Demon" is derived from the Greek δαίμων (daimōn), which were spiritual beings in Greek and Roman mythology that were considered to be more powerful than mortals, but less powerful than the higher gods, such as the Olympians. In the original Greek usage of the term a "daimon," or "daemon" in Latin, described all such spirits, whether they were beneficial, harmful or neutral. As Christianity spread throughout Europe, Christians began translating Jewish and early Christian texts into Greek. In these texts multiple Hebrew words were translated into "daimon," with these words typically being related to evil or unclean spirits. At the same time Yahweh's messengers were translated to Greek as άγγελος (ángelos), which developed into the modern "angel."
Demons in Judaism:
In Judaism, what is translated to English as "Demon" can be one of a multitude of evil or unclean spirits. Contrary to Christianity, these demons are not always considered to be fallen angels. The exact views on the nature of demons has varied widely across both Jewish history and across the various Jewish sects that exist today, which has lead to extensive discussion and debate.
See: Demons in Judaism
Demons in Christianity:
As mentioned above, demons in Christianity are almost universally considered to be fallen angels that rebelled against Yahweh.
Typically Christians consider all deities other than Yahweh to be either empty idols, or demons posing as gods. Because of this, Christian lists of formal demonologies will often include gods, goddesses, and mythological creatures from other religions around the world that have been "demonized."
Demons in Islam:
Though fallen angels can sometimes be seen as demons in Islam, more commonly demons are considered a separate species from Angels, called Jinn. Jinn are thought of to be made of fire, can eat, drink, sleep, and reproduce, and have free will. Angels in Islam are often thought to be made of light, and do not need to eat, drink or sleep, cannot reproduce, and lack free will.
See: Demons in Islam
Demons in Hinduism:
In Hinduism the Asuras are beings who oppose the Devas (the gods). Both the Devas and the Asuras fought over the amrita, the elixer of mortality. The Devas and the Asuras are in a never ending conflict.
See: Demons in Hinduism
Demons In Buddhism:
In Buddhism, demons are seen as forces that prevent humans from reaching Nirvana. The most notable of which is Mara, the tempter.
See: Demons in Buddhism
Demons in Shinto:
In Japanese Shinto, Yōkai (a class of strange supernatural beings) can sometimes be translated to English as "Demons," but can also be translated to monster, spirit, or goblin. The most notable creature from Japanese culture that's considered to be a demon is the Oni, though the term is also often translated to ogre. The oni are large, monstrous, violent, and cruel.
See: Demons in Shinto