Just my take...

Spirit is the part of us that is intimately connected to the ground of all being. To be spiritual is to acknowledge that such a connection exists and to make an effort to directly discover the nature of that connection.

As pointed out above, referring to Buddhist and Taoist practices, it is not necessary to attempt to define or assign a set of attributes to the ground of all being itself. Neither Buddhism nor Taoism adheres to the concept of a defined "supreme being". It is sufficient to merely admit to the existence of something that transcends our understanding. In fact, most true spiritualists who come from a conventional western religious background (ie Thomas Merton, Martin Buber) eventually adopt a far more metaphysical concept of "God" than generally held by others of their faith.