In most European paths (even more), spirit is that which we are all made of. Nothing can be physical without also being not physical (energy - etheric - subtle). Spirit pervades everything, which is why it's never associated with anything like a color, animal, direction, deity (directly). It is what most of us believe to be how Magick works, essentially.
When one casts a circle (as seen in these European traditions), one uses the elements and their forms to cleanse and form it. Most use incense for air and fire and a salt water mix for water and earth. Our own energy is what is used for spirit. Ourselves for example.