According to Webster's New World Dictionary, an incubus is a spirit or demon thought in medieval times to lie on sleeping persons, especially women, with whom it sought sexual intercourse. See succubus. This myth has been around long before the advent of Vlad Tepes.
My own personal thought is that this creature was invented to explain men and women waking from erotic dreams. It was simple to explain an erection or wet spot to your spouse in the morning by claiming you were demonically possessed by an incubus or succubus (the female version). Such a horrible beast would be easy to detract from the shame of dreaming about sex during such a time when it was considered sinful enough for punishment.
In modern times, it is usually thought of as one who feeds from sexual energy, the way a psychic vampire feeds from chi, or a sanguine vampire feeds from blood. The general perception is that the incubus or succubus feeds off either the energy created from sexual tension, or the energy created by sexual activity.
As I define myself, I see it in a much different light. I find it to be much more spiritual and definitive in nature.
At the start of the Taoist studies into alchemy, it was believed that, to become immortal, one must consume the "Pill of Immortality". In ancient China, the "Pill" was widely sought after. The Great Wall was built to keep outsiders from coming into the country to steal it. Emperors demanded that their best alchemists try an create such an item. Though previously thought that such an item was only created by the gods, the concept was eventually translated to a physical, man-made creation. It was first tried as a combination of gold and cinnabar. Found to be toxic, many variations were attempted in the hope that eventually they could be eternal.
Over centuries, Taoism developed an enormous deviation of beliefs, or Schools. One of those schools was the Lung Men Sect. A branch of the Lung Men developed an idea. The thought was that gestative fluids, those fluids created in a woman to create, develop, and nurture a life in their womb, held the answers to sustained life. My personal beliefs follow much of the same thought.
Orgasmic fluids of a female create the lubrication, and foster the heightened temperatures for conception to initiate life. Menstrual fluids are produced by the body to flush out the unused vitamins and nutrients contained in the blood in the womb and cervical area so they are fresh on a cyclical basis. I am a consumer of both.
Is it the "Pill of Immortality"? Is it the fountain of youth? This is probably not going to replace anti-wrinkle cream or plastic surgery. I am not in a position to say definitively, one way or the other. What I do know is that, for me, I have more stamina, and feel more invigorated when I consume. It might not work for most, but I have no complaints. My chi feels more complete, and I cannot explain why it does. It does give me some reassurance that menstrual blood is now being utilized by some for stem cell research. Some, even in our own community, think of it as "dead blood".
This doesn't mean that all incubi feed the same way, or that succubi all feed from male sexual fluids. We are all intrinsically the same in that sexuality is a common denominator. We are, also, all different in that we have some inherent deviation in us that keeps us from being clones of one being. I am not an advocate of exact duplication.
This definition is not to replace what is commonly thought of by most. This is a personal interpretation. I add this not to be the ultimate scholar, but rather as food for thought as you walk your own path and define yourself. One definition of anything is not appropriate when it comes to groups of beings, and definitions are only as prudent as the individual assessing them.
This what makes me a bit different. This, in part, is what makes me who I am. This is my path as others come to know me, and in time, know themselves.