Phew! (I must be determined to have some argument here!)
But I have been receiving hints on this question for some time, starting with Wink and the intractability of institutional Policies & Personalities, digging up some wonderful stuff by William Stringfellow, who had a somewhat different bestiary of Powers & Principalities--and a slightly different conception of them. "Paul"--that is, the man who wrote 'Ephesians'--gives us only a hint--And what we see of Palestinian demons in their native habitat, being exorcized by Jesus, is quite a puzzle to a Modern (or "Post-modern," or even to a "Postpost-modern" (mine, for example?) mind.
Okay, a "demon" is something like a subroutine!
That is, it's something like a program a human mind can execute, much as a computer follows a structure of machine instructions.
Since no one has commented yet, let me go into more detail
There are many possible starting points; I like Stephen Gaskin's way of saying that we are all One: that we are all, at some level we don't typically notice, telepathic.
Jung spoke of a "Collective Unconscious"--the sort of mental process people sometimes do without conscious attention, ie in adding two numbers or in locking a door on leaving work--happening equally automatically, without the supervision of any one individual mind, but dealing with vaster questions, whether for a tribe or for a nation or for all sentient beings. (Jung wasn't concerned about postulating any physical structure to carry out such computations, nor need we be. Something connects us, at different levels of coherence, benevolence, universality for different people, much as our physical environment is dense with radio signals, while any one radio receiver will be tuned into one of them at a time. & by analogy, the highest manifestation of God we can relate to would be making a sort of vast background hum connecting it all--to speak muy figuratively, but not allegorically about this!)
So one could think of "a spirit" as a subroutine running on the Collective Unconscious.
The original meaning of "daemon," as we all know, was morally indeterminant. For an ancient Greek this might be anything from a pixie to a god, benevolent or merely dangerous. The first Christians, being Jews and hostile to small-g gods, started with the assumption that the spirits the Greeks knew must be evil beings, rivals for the devotion rightly due to God. But angels, in the Jewish scheme of things, were formerly imagined as much the same sort of entity, whether they were the unruly angels of nations hostile to Israel, or angels properly engaged in their God-given duties.
We first find stories of people possessed by "demons"--Where? In 1st Century Judea, among a pious Jewish population suppressed under the Hellenized power of Rome. Those foreign devils get into native Jews, who suffer conflict because they can neither accept the ways of the Gentiles nor entirely fend them off. And Jesus, somehow, heals the confusion, sending the foreign spirits packing! By his sheer personal authority, in the gospel accounts--but I think those writers were as mystified by how he did it as we today are mystified by the stories themselves!
So what would it mean to torment "the Devil" forever? How does one torment a subroutine, anyhow?
"The Devil" is described as "the god of This World." What world? The world of physical fears & desires, of force & strategems for grabbing what one wants & enjoying all one can get of it. For a dinosaur, this is a perfectly adequate religion. For a human being, not so good!
But that dinosaur-level is the level of moral-ethical development we find in nation-states, corporations, and religious institutions. An institution under the rule of Christ, as Stringfellow imagined, would take no thought for its own perpetuation, merely seek to carry out the will of God even at the risk of bankrupcy! What we see driving a typical human institution is otherwise.
So if that "whatever it takes" spirit were to be displaced by a different spirit, a spirit of loving our brother _as_ ourself... as he is, like it or not!...