The Hebrew noun kopher means a ransom paid to cover guilt or avert punishment. It can also refer to pitch (the waterproofing material used on Noah's ark), revealing the related idea of covering. From the root kaphar (to cover, atone).
Kopher is the price of a life — the ransom paid when a guilty party deserves death. In Exodus 21, a ransom (kopher) is paid for life that was endangered. The theological weight is immense: God's forgiveness requires a ransom. Ultimately, Christ is the final kopher — 'a ransom for many' (Mark 10:45). The related word kaphar undergirds the entire sacrificial system.