Spirit is indeed an element, and it does have connections to all other elements, but not quite as strongly as you may be thinking.
Let me explain... The pentacle has 5 points on it, each of them a representation of one of the governing elements. Fire, Earth, Air, and Water, along with Spirit at the northern point.
Spirit is the representation of "self", and is the most "you"-centric of all the elements, and it's strength varies depending on your strength of will.
Any normal person is capable of developing a connection to the other 4 elements at any time through perseverance and strength of will. However, everyone is born with an innate connection to "spirit", because it's what defines all of us.
It's hard to give a concise description of exactly what it is because it varies so much from person to person, but I hope what I've written helps you understand a bit better