|Gather 'round the campfire|
Spirits are purported disembodied beings — wandering minds without the physical connections — pure intelligence or thought. Many religions see them as what remains after one has died. The idea of spirits may have arisen from the awareness that something seemingly "non physical" and distinct from the body is what animates us. If one cuts off my foot, or my arm, or any part of the physical me, there is still something left. Some cultures define that essence as a soul, others a spirit, and still others as both. This duality, of physical body and 'soul' leads directly to the belief that consciousness will somehow continue even after the very organ of consciousness, the brain, has turned into worm-ridden tapioca pudding.
Most cultures posit some form of invisible spirits which may or may not be benevolent, but which generally all share the same characteristic of being an invisible friend or foe, that's not quite a deity, but not quite mundane.
In Philosophy and Theology
The Israelites of the Old Testament did not postulate the survival of the identity after death until the injection of Hellenistic ideas, especially Plato's "ideal forms". Just as an actual chair was held to be a poor copy of a perfect "ideal" chair which existed somewhere in a remote mathematical mindscape, so was the psyche held to be the ideal, "true" human being, temporarily imprisoned in a corruptible body. The distinction between the mind and the body — and sometimes the dichotomy between the evil body and the good soul — made their way into Jewish communities, first in Jewish colonies outside of the Levant, bleeding into Judea during the 2nd and 1st centuries BCE.
An Alexandrian philosopher named Philo, developed the idea of the "Logos" — that is, the idea that God's word is itself an ideal, spiritual being. This idea must have gained attention from early Christians, for the Gospel of John is loaded with references to this "Logos" (which, of course, the author identifies with Jesus Christ). Paul of Tarsus, in turn, developed Christian spiritual theology, emphasising the superiority of the spirit over the body, and often blaming "the flesh" for causing problems in the early Christian communities. However, Paul still holds onto a very Jewish distaste of the body and soul being separate.
René Descartes, the father of modern philosophy and skepticism, divided the world into matter (physical things) and mind (immaterial things that move the physical world). As a result of his radical doubt, he made the observation "cogito, ergo sum" (I think, therefore I am) — identifying himself with his mind, which he considered unchanging, and his body, which could change.
A "spirit" in alcohol terms is used to refer to a type of alcohol that is low in sugar but usually around 35% ABV. Some examples include gin and vodka.