Catechesis is basic Christian religious education of children and adults, often from a catechism book. It started as education of converts to Christianity, but as the religion became institutionalized, catechesis was used for education of members who had been baptized as infants. As defined in the Catechism of the Catholic Church, paragraph 5 :

Catechesis is an education in the faith of children, young people and adults which includes especially the teaching of Christian doctrine imparted, generally speaking, in an organic and systematic way, with a view to initiating the hearers into the fullness of Christian life.