What is Spirit? Can we define it? The simple answer is no. Spirit cannot be defined, it simply is.
From my experience, to know Spirit, one must be spiritual. Being spiritual is a state of being arrived at by connecting to Spirit through developing higher awareness through such practices as meditation, yoga, chanting, communing with nature, fasting, contemplation, silencing your mind. This higher awareness creates an inner clarity in which you can feel the presence of Spirit. The secret is that Spirit is always here and it is our awareness that perceives Spirit or not.
You cannot think Spirit into presence, you can only feel it within. What is this feeling that you get when you recognize Spirit's presence within? You feel an overwhelming feeling of Love, peace, happiness and well being. There are no thoughts. You may see images of doves, angels, ascended masters, avatars and those kinds of sacred images that come to you. It is an allowance that Spirit is real and that you are a form of Spirit.
Being Spiritual is not an ego trip. It is not grandiose but it does contain grandeur. You don't have to go around telling people how Spiritual you are. People will be able to feel you and say something like, "You seem comfortable in your own skin."
Being Spiritual is not doing anything, it's being something and that something is simply connecting to your own spirituality within. Once connected within yourself, you commune with it in everything.