There are many meanings associated with the noun spirit, such as supernatural being, air or essence, Holy Spirit, psyche, soul, ghost, as well as the disposition of a person or thing. Evidently the word spirit has evolved to encompass many aspects of life, and its meaning has become diluted or diffused by the many uses in various languages and cultures. The only ghosts ever "seen" are the ones in our dreams and mythologies and those we now categorize in medical science as hallucinations, which can be associated with drug use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens. However, underlying all of the various definitions is the intuitive understanding that spirit is a non-physical potential that is indirectly observed; an animating force or essence of vitality that we can "point at" but cannot observe directly—much like gravity.
The word spirit actually originates from the Latin spiritus meaning breath. Interestingly, this word is also associated with the word animate which is derived from Latin animus meaning spirit, and from Latin anima meaning life or soul. In turn, these are derived from the Sanskrit word aniti meaning breathes. Evidently, spirit is the animating potential that induces breathing and enables us to differentiate a living thing from non-living matter; it is the essential life-giving principle that furnishes awareness at both a conscious and unconscious level. What is not evident is the fact that this animating potential is not just localized but universal. It is what modern science has termed "dark energy" and ancient Eastern tradition has termed "Akasha". This potential is like a pulsating vibratory brain that permeates everything in the universe.
Since prehistoric times, we have tried to describe or explain the life force of the universe using spiritual references and representation. Consequently, the underlying theme of primitive mythology and religion was always spirituality. Hence, whether our spiritual beliefs lead us to consciousness, the Buddhist immortal Void, Atman/Brahman, God, Holy Spirit, or any of the other names and forms we use to represent the ultimate reality, all of these point to a singularity that is best represented as spirit. Ultimately, it is not the illusive grasp of the conception of spirit but a deeper sense of its omnipresence that really matters.