Specifically, Spirit is that persona which embodies the emotional expression of God in response to God's conceptions. It was this aspect of Spirit that responded to the Second-Generation Creation when God created, and saw “that it was good”. Spirit is linked with the expression of emotion in more human terms, but functions on levels far beyond that.
Following creation, and the sacrifice of the Word incarnate, the Spirit persona, in some undefined manner, also separated out from God to actually intertwine and merge with various living entities of the Second-Generation Creation. This was for the apparent purpose of nurturing closer relationships between those entities and God as a whole. The exact nature and extent of this symbiosis, both in terms of degree in any given individual and the number of beings so bound, remains unknown.
This was then the basis for the Trinitarian aspect of God. God is also often referred to as the Creator-Word-Spirit in honor of this fact.
Spirit is also an alternate term used for essence, or the inner nature of a being.
- Wikipedia Article on Spirit