The act by which God takes us as His own children and makes us heirs to the happiness of heaven.
Unlike natural or legal adoption which alters the standing of the adopted one externally or socially, supernatural adoption affects our very life by transforming our soul into the likeness of Jesus Christ and making us His co-heirs to the kingdom of heaven.
This is repeated in various terms, as by Saint Paul in his Epistle to the Romans, 8.15, "You have received the spirit of adoption of sons, whereby we cry: Abba (Father)," and by Saints John, Peter, and James.
It occurs frequently in the early liturgies and the Fathers stress the fact of our deification, i.e., of our God-given and godlike life by virtue of this adoption which is the result of sanctifying grace.
New Catholic Dictionary