one, peace, quietness, rest, + set at one again

Concise Bible Dictionary:

This term is used to express the present attitude and testimony of God toward man consequent on the declaration of God’s righteousness in the death of Christ. The state of man which was obnoxious to the holiness of God by reason of sin has been removed in the cross. Hence the believer is justified by faith, and has peace (peace of conscience) with God through the Lord Jesus Christ (Rom. 5:1). Christ made peace through the blood of the cross (col. 1:20); and to the Christian God is “the God of peace,” and the Lord Jesus is “the Lord of peace.” He also is peace between believers, having on the cross broken down the barrier between Jew and Gentile (Eph. 2:14-15).
When the Lord Jesus left the earth He left to the disciples peace, and said, “My peace I give unto you.” Peace is also spoken of as the state of heart in which a believer is kept in regard of circumstances. The record in the Old Testament is, “Thou wilt keep him in perfect peace (peace, peace, margin) whose mind is stayed on Thee, because he trusteth in Thee” (Isa. 26:3). The Christian makes his requests known unto God, and the peace of God that passeth all understanding keeps his heart and mind through Christ Jesus—peace of heart (Phil. 4:6-7). Blessed privilege! and what a contrast to “There is no peace, saith my God, to the wicked” (Isa. 57:21). The Lord Jesus will, in the future, among His other titles, be hailed as PRINCE OF PEACE (Isa. 9:6).

Strong’s Dictionary of Greek Words:

Greek:
εἰρήνη
Transliteration:
eirene
Phonic:
i-ray’-nay
Meaning:
probably from a primary verb εἴρω (to join); peace (literally or figuratively); by implication, prosperity
KJV Usage:
one, peace, quietness, rest, + set at one again