Holman Bible Dictionary 
go'el Leviticus 27:13-31 Exodus 6:6 Job 19:25 Psalm 103:4 Isaiah 43:1 Isaiah 41:14
The human avenger is tied closely to the institutions of cities of refuge, land ownership, and levirate marriage. Cities of refuge offered people who killed without intention or hatred a place of escape from the avenger of blood ( Exodus 21:12-14; Numbers 35:6-34; Deuteronomy 4:41-43; Deuteronomy 19:1-13; Joshua 20:1-9 ). The human go'el may be a brother, an uncle, a cousin, or another blood relative from the family ( Leviticus 25:48-49 ). An established order among these determined the one legally responsible to act as go'el ( Ruth 3:12-13 ). The avenger or go'el is responsible to take the life of one who killed a family member ( Numbers 35:12 ), to receive restitution for crimes against a deceased relative ( Numbers 5:7-8 ), buy back property lost to the family ( Leviticus 25:25 ), redeem a relative who sold himself into slavery ( Leviticus 25:48-49 ), or marry the widow of a relative without sons and perpetuate the family ( Deuteronomy 25:5-10 ). Avenging the death of a relative is placed under strict limits. The murderer must have intentionally waited to kill the relative ( Exodus 21:13 ) or willfully attacked the relative ( Exodus 21:14 ). Vengeance could be exercised only before the murderer reached the city of refuge or after the court either at the victim's hometown or at the murder site judged the case ( Numbers 35:12 ). The avenger was free to act if an iron object was used to commit the murder ( Numbers 35:16 ), or if a stone or wood object was used ( Numbers 35:17-18 ). Pushing a person to death because of hatred made one liable to the avenger ( Numbers 35:20-21 ). Unintentional acts could not be punished ( Numbers 35:22-24 ).
A killer judged to have committed the crime without hatred or intentional planning was sent to the city of refuge until the death of the high priest. The avenger could not touch the killer in the city of refuge, but if the killer left the city of refuge for any reason, the avenger could reap vengeance even against the unintentional killer ( Numbers 35:22-28 ). This shows that even unintentional murder involved sin for which a penalty had to be paid. The law of the avenger thus prevented the shedding of innocent blood while also purging the guilt of murdering the innocent ( Deuteronomy 19:11-13 ). The law maintained the reverence for human life created in the image of God ( Genesis 9:5-7 ).
The New Testament sets up government to avenge evil doing ( Romans 13:4 ), while noting God's role in avenging wrong against a brother ( 1 Thessalonians 4:6 ).
Hawker's Poor Man's Concordance And Dictionary 
Particular mention is made in Scripture of the avenger of blood, ( Deuteronomy 19:6) and cities of refuge were appointed for the manslayer. ( Numbers 35:12; Joshua 20:5) There is much of Christ as a refuge, represented under this appointment, and we shall do well at any time when reading those Scriptures, to be on the look out for discoveries of the Lord Jesus in the several features of the history. Every man, by sin, is a murderer, yea; a soul murderer, and that of himself. And the avenger, both in the law of God and the justice of God, is always, it may be said, in pursuit of the sinner, until he hath taken shelter in Christ. Jesus is the city of refuge. And Jesus is near to flee unto. ( Hebrews 6:18) It was not the stronghold of those places which secured the manslayer, but because it was the provision of divine mercy. "Salvation the Lord appointed for walls and bulwarks." ( Isaiah 26:1) And what endeared the city of refuge to the manslayer was, that it was wholly of God's own appointing. And the general and extensive nature of its security was, that poor stranger, as well as the Israelite, found a sanctuary. ( Numbers 35:15) Such is the Lord in the greatness and extensiveness of his salvation. "For (saith the apostle) there is neither Jew nor Greek, neither bond nor free, male nor female, for they are all one in Christ Jesus." ( Galatians 3:28) Sweet type of Jesus, the city of refuge.
Webster's Dictionary 
(1): (n.) One who takes vengeance.
(2): (n.) One who avenges or vindicates; as, an avenger of blood.
King James Dictionary 
AVENG'ER, n. One who avenges or vindicates a vindicator a revenger.