The Greek word aichmalōtos means a captive or prisoner of war — one taken by the spear (aichme = spear, halōtos = taken). It was used of those captured in battle and carried off as slaves or hostages. In Luke 4:18, Jesus applies it to spiritual captivity that the Messiah comes to release.
The most theologically crucial use of aichmalōtos is in Jesus' reading from Isaiah 61 at Nazareth (Luke 4:18): 'The Spirit of the Lord is upon me... to proclaim liberty to the captives.' This is Jesus' mission statement for his entire ministry: the reversal of spiritual and physical captivity. Paul expands the image in Ephesians 4:8 (quoting Psalm 68:18): 'When he ascended on high he led a host of captives.' Christ, having conquered death and sin, leads captive those powers that once held humanity in bondage. The same word describes literal prisoners in Acts (e.g., 2 Timothy 1:8) and the spiritual captivity of false teaching in 2 Timothy 3:6. Freedom from captivity — purchased at the cross — is a core metaphor for the gospel.