Avenger of blood - (Easton's Bible Dictionary)
(Heb. goel, from verb gaal, "to be near of kin," "to redeem"), the nearest relative of a murdered person. It was his right and duty to slay the murderer (2 Sam. 14:7, 11) if he found him outside of a city of refuge. In order that this law might be guarded against abuse, Moses appointed six cities of refuge (Ex. 21:13; Num. 35:13; Deut. 19:1, 9). These were in different parts of the country, and every facility was afforded the manslayer that he might flee to the city that lay nearest him for safety. Into the city of refuge the avenger durst not follow him. This arrangement applied only to cases where the death was not premeditated. The case had to be investigated by the authorities of the city, and the wilful murderer was on no account to be spared. He was regarded as an impure and polluted person, and was delivered up to the goel (Deut. 19:11-13). If the offence was merely manslaughter, then the fugitive must remain within the city till the death of the high priest (Num. 35:25).