Baptism (from the Greek noun βάπτισμα báptisma) is a Christian rite of admission and adoption, almost invariably with the use of water.  For Catholic Christians, Baptism serves as the first sacrament one receives when entering the Faith. It is a Sacrament of initiation, which means it can only be received one time. The recipient receives justifying and sanctifying grace when baptized, and the Holy Spirit begins to dwell within them. 

 

If someone who has been baptized in another Christian denomination wants to join the Catholic Church, then the Church will accept that baptism as long as the Trinitarian formula was used, which is the case in almost all baptisms.