sanatio in radice
(Latin: a healing at the root)
A secret validation of an invalid marriage without securing a renewal of consent.
This sanatio can be granted only by the Holy See; and there is no validation of this kind for a marriage which has an impediment of the natural or Divine law, but only for a marriage which was null because of some ccciesiastical obstacle.
In other words, the Church can undo what it has done; and this action may retroact, so that, by a sort of legal fiction, the marriage will then be considered to have been legal from the beginning; it is especially useful in the legitimation of children already born of such a union.
New Catholic Dictionary