Canonization (also spelled Canonisation) is the act by which a Christian Church declares a deceased person to be a saint, inscribing that person in the canon, or list, of recognized saints. Originally, individuals were recognized as saints without any formal process, as happened, for instance, in the case of Saint Peter and the Blessed Virgin Mary.
In the Catholic Church (excepting the Eastern Catholic Churches), the act of canonization is now reserved to the Holy See and occurs at the conclusion of a long process requiring extensive proof that the person proposed for canonization lived, and died, in such an exemplary and holy way that he or she is worthy to be recognized as a saint. The Church's official recognition of sanctity implies that the persons are now in heavenly glory, that they may be publicly invoked and mentioned officially in the Liturgy of the Church, most especially in the Litany of the Saints in the Canon of the Mass. Other Christian Churches still follow the older practice (see, for instance, below on Eastern Orthodox practice).
Canonization, whether formal or informal, does not make someone a saint: it is only a declaration that the person is a saint and was a saint even before canonization. It is generally recognized that there are many more saints in heaven than have been canonized on earth.