A common philosophical definition describes spirit as "the life force of living beings." We occasionally use this same concept when we talk about groups like student bodies or athletic teams. Without spirit, there is no enthusiasm, no life.
Spirit can also mean the real sense of significance of something. The expression "the spirit of the law" reflects this meaning; so does the expression "the spirit of '76.'
In a religious sense, we often refer to God as Spirit. The New Testament has many references to the Spirit in connection with Jesus and his followers. Jesus is conceived by the Holy Spirit; God's Spirit descended on him at his baptism; and the Spirit leads him into the desert. Jesus was filled with the Spirit as he preached and performed miracles. Jesus himself promised to send his followers the Spirit of truth who will glorify him.
Our bedrock Christian belief is that the Holy Spirit is the third person of the Blessed Trinity, of the same "substance" (nature) as the Father and the Son. The Holy Spirit is the breath (Spirit) of the Father with whom Jesus is totally filled. The Spirit is active in the Word and is the gift given by the Father and Son to all of Jesus' followers.