Circumcision is a physical operation that entered one into the Abrahamic covenant in the Old Testament. It was instituted by God (Gen. 17:10-14) and performed on the eighth day after birth (Luke 1:59) by removing the male foreskin. It was a sign of the covenant God made with Abraham (Gen. 17:12, Rom. 4:11) by which people could be associated with national Israel.
In the New Testament, the physical operation is not practiced. Instead, a circumcision of the heart of the Christian is taught (Rom. 2:29, Col. 2:11-12) and considered to be true circumcision (Rom. 2:29).