To administer the sacrament of baptism to someone, typically by sprinkling or immersing them in water. The priest will baptise the child during the Sunday service.
To give a name or nickname to someone or something, often in a ceremonial way. The new ship was baptised with a bottle of champagne.
To initiate someone into a new experience, especially something challenging or difficult, often expressed in the phrase “baptise by fire.” The recruits were baptised by fire on their first combat mission.