To dip, plunge, immerse, or wash. In secular Greek, it described dyeing cloth (immersing it in dye) or a sinking ship. John the Baptist adopted it for his ritual of repentance. Jesus transformed it into the initiatory rite of the new covenant community, commanding baptism in the name of the Father, Son, and Holy Spirit.
Baptism in the NT carries multiple layers: identification with Christ's death and resurrection (Romans 6:3-4), washing of regeneration (Titus 3:5), entrance into the covenant community (Acts 2:38), and reception of the Holy Spirit (Acts 1:5). The physical act of immersion enacts the gospel visually — going under (death) and coming up (new life). It is the believer's public declaration that the old self has died.