Aliens, which are described as hypothetical extraterrestrial life in Encyclopedia Britannica, are not considered demons. Demons are defined as supernatural beings who mediate between the spiritual and material worlds. In ancient times people believed demons were not supposed beings, but real. Many still believe this today.
Aliens are typically conceptualized as highly intelligent creatures from outer space capable of exhibiting advanced thought and purposeful activity. Scientists are searching for radio or light waves from other solar systems within our galaxy that might suggest the actual existence of extraterrestrial life, but have yet to detect such signals.
Demons, unlike aliens, are more often associated with religion than science. Throughout history different religions have to one degree or another attributed various powers and phenomena to these sacred, holy or transcendent creatures.
In Christianity, demons are viewed as former angels who rebelled against God, resulting in their being thrown out of heaven and labeled as evil due to their unholy deeds. Likewise, Buddhists of Sri Lanka also believe in malignant spirits capable of causing disease, dementia or misfortune.
The Bible reports numerous instances in which demons entered into people, causing mental or physical maladies or inducing them to exhibit bizarre behavior. It also prophesies a time in the future when demonic spirits will perform miracles and gather together rulers of the world to battle against God.