The term "Holy Ghost" is in the KJV 89 times in the New Testament and zero times in the Old Testament. This tells me it is something that is related to Jesus Christ in the flesh that God wants to highlight and have students of the word to meditate on and to search out. There are hints of what he wants us to know via that terminology just in some of the verses he gives us.
He is related to the physical person of Jesus Christ and is distinct from him (as a fleshly person).
Matthew 1:18
Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the
Holy Ghost.
Soul and spirit. The human soul associated with the body and the Holy Ghost is God in the flesh.
No man born of a woman had ever had God in his body from his physical birth until Jesus Christ, conceived in the womb of Mary by the Holy Ghost, and born in the image of God, a trinity, body, soul, and Spirit of God. In this way he is equally God and Man in one entity called Jesus, "God with us." The necessity of having himself in the bodies of of men defines reconciliation with God and is a theme of his ministry to Israel, to whom he came and through whom this will be initiated for fallen men. See John 6 in this frame of reference. In Romans 8, much farther down the line of typical teaching and revelation of the mystery of faith Paul would say, now if any man have not the Spirit of Christ he is none of his. Receiving the Spirit of Christ requires he pour IT out and that men receive it by believing the gospel, which is the death, burial, and resurrection of the body of Jesus Christ, him being glorified by the Spirit that raised it from the dead by indwelling it and giving it life. The Holy Ghost is the gift of God because he is the Spirit of Christ. When he, as the gift, is received into the body by faith then those whom he indwells become trinitarian by his presence in their bodies, which is likened to his temple.
Romans 8:10 And if Christ be in you, the
body is dead because of sin; but the Spirit is life because of righteousness.
John 7:39 (But this spake he of the Spirit, which they that believe on him should receive: for the
Holy Ghost was not yet given; because that Jesus was not yet glorified)
ohn 16:7
Nevertheless I tell you the truth; It is expedient for you that I go away: for if I
go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.
One can now understand why he is likened to a Ghost. He is in the body of the saint but exists outside of it as well. Not just any ghost but the Holy Ghost.
The Holy Ghost is a NT doctrine for obvious reasons. He has the characteristics of a ghost in his character.
That is my take and the KJV translators probably did not know any of these truths and so they identified the Holy Ghost in the NT some other way.
Jn 1: 4 In him was life; and the life was the light of men.
Past tense verses
33 He that hath received his testimony hath set to his seal that God is true.
34 For he whom God hath sent speaketh the words of God: for
God giveth not the Spirit by measure unto him.
Present tense verses
35 The Father loveth the Son, and hath given all things into his hand.
36 He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.
He has given life to believers in the person of the Holy Ghost, which is the Spirit who dwelt in the body of the Son.