The English word "baptism" is from the Greek term βάπτισμα (baptisma), which means "immersion." This term is used throughout the New Testament to describe a ritual where new followers of Jesus are submerged in water in order to publicly identify with Jesus, renounce their former way of life, receive the Holy Spirit, and enter into the community of other Jesus-followers. 

To learn more, you can read the article on Baptism and the Lord's Supper in the Baptist Faith & Message 2000.