opus operatum; ex opere operato
A technical phrase used by theologians since the 13th century to signify that the sacraments produce grace of themselves, apart and distinct from the grace dependent upon the intention of the person conferring the sacrament; the latter effect is designated by the phrase ex opere operantis.
The phrase is first found in the writings of Peter of Poitiers (c.1130-1215),
"The act of Baptism is not identical with Baptism because it is an opus operans while Baptism is an opus operatum."
The phrase was not in general use in the time of Saint Thomas but it was officially adopted by the Council of Trent and used to signify the objective character of the sacraments as producers of grace in opposition to the subjectivism of the Reformers.
According to Trent, therefore, the term opus operatum signifies that the correct use of the sign instituted by Christ produces the grace irrespectively of the merits of either minister or recipient (ex opere operantis), though the intention of conferring the sacrament is required in the minister and the intention of receiving in the recipient, if he be an adult, for a valid and worthy reception of the sacrament.
For the council clearly states that the sacraments "confer Grace on those who do not place an obstacle thereunto."
New Catholic Dictionary