“Demon” is a generic catch-all term used to describe illegal genetic mutants. Most commonly, these beings are refugees from Hell who didn't manage to beat the nano-plagues. Demons tend to resemble monsters - many of the rogue nano-modifiers included combat modifications, so fangs, horns, claws and dermal plating are not uncommon. Given the Darwinian competition among nano-plagues, the specific mutations of a particular demon is often unique. Generally, any descendants of a demon will be one as well, though the child's mutations may be different.
Demons are illegal in the Empire of America, and as such have no legal rights, and are exempt from any and all protections granted by the Constitution. Demons are subject to an immediate death sentence under the law. The reasoning for this is to prevent the sort of wild nano-plagues that have turned Europe into Hell from gaining a foothold in North America.
While the purpose of the laws on Demons is to keep the wild monsters of Europe out, those same laws are also used to control genetic and implant research. The Science Police can and do use the laws as grounds for cracking down on domestic research done by private firms.
Some Demons do manage to live in America, but they live under the constant spectre of capture by the Science Police.