Wikipedia - Spiritual formation may refer either to the process and practices by which a person may progress in one's spiritual or religious life or to a movement in Protestant Christianity that emphasizes these processes and practices.
Dallas Willard - Spiritual formation in the tradition of Jesus Christ is the process of transformation of the inmost dimension of the human being, the heart, which is the same as the spirit or will. It is being formed (really, transformed) in such a way that its natural expression comes to be the deeds of Christ done in the power of Christ.
Richard Foster - The Christian idea of spiritual formation is, very simply, the formation and conformation and transformation of the human personality—body, mind, and spirit—into the likeness of Jesus Christ.