This is a complex, multi-sided question but I intend to keep my answer simple. For the purposes of this blog (my online BOS) it will be important to develop a definition. This is by no way an unquestionable or unwavering definition and I would love to have feedback or impressions.
First it is important to understand that things can seem demonic or appear demonic when there is an entirely different explanation. Sometimes the explanation is psychological and scientific like mistaken identity or sleep paralysis. Other times the cause can be almost as supernatural like discarnate entities or magicians traveling astrally.
For my purposes and to remain general I will describe a demon as follows: There are many types and varieties, legion to be exact. In a ever present facet they are base archetypes, avatars or atavisims of human desires and emotions run amok. Generally speaking they are the macrocosmic reflections of the microcosmic battles happening within your soul and mind. These battles are very real for victims and this can be called demonic oppression. At times they can grow more present or closer to our own plane of existence. Some have evolved to incorporate higher integration to further their base nature. Any desire taken to exremes can overwhelm the person in a very physical sense leaving the person feeling powerless and unable to focus on anything but the torment. In many instances this constant barrage leads to dark collaboration with the demonic entity called demonic obsession. It is important to note that I believe almost any idea, taken to “absolute truth” will manifest as a serious demon. In more extreme cases when the demon is very powerful it can possess an individual. This demonic possession occurs in an enrapture of the demons sphere of influence. Like being filled with lust or as ritualistic possession where the monster is drawn up and takes over. A demon is grown through repression, neglect, denial, obsession and other shadows and manipulate through secret and urges. The desire is the source and the solution. It is important to recognize the strange duality that exists between desire and fear. I would venture to say that fear and desire or just two forms of the same thing. A smaller part of the Great Work involves the naming and binding of the demonic princes within our psyche, thereby becoming the unquestioned god of your microcosm.