In Buffy, you see hundreds of things referred to as "demons" or "demonic", and many other things (Trolls, Werewolves, Witches, etc) are not. But what actually counts as a demon in the Buffyverse?
Typically, anything from another dimension that isn't powerful enough to be called a god. Also, good looking extra-dimensional beings are sometimes not called demons.
A human who has been in another dimension and been altered has been called a demon, I believe.
Remember that Anya was/is a demon.
The term demon is specifically defined in the Buffy the Vampire Slayer Encyclopedia: The Ultimate Guide to the Buffyverse.
Supernatural beings native to hell dimensions. Generally they lack souls and are considered evil. They often possess great strength and endurance, as well as powers like teleportation or psychokinesis
It follows that anything that doesn't fall into this category is some sort of hybrid or simply mis-described.