It means Holy Spirit. “Ghost” is Anglo-Saxon for spirit, “spiritus” in Latin meaning a breath. Thus the word spirit is associated with human breathing as a kind of intangible impulse. Christ used the term to bring home to us that the Third Person of the Holy Trinity is the impulse of love invisible and intangible between Father and Son. Since love tends to union, and union with and in God is holiness, the Third Person is termed the Holy Spirit.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty