...a sacrament properly speaking is that which is ordained to signify our sanctification. In which three things may be considered; viz. the very cause of our sanctification, which is Christ's passion; the form of our sanctification, which is grace and the virtues; and the ultimate end of our sanctification, which is eternal life. And all these are signified by the sacraments. Consequently a sacrament is a sign that is both a reminder of the past, i.e. the passion of Christ; and an indication of that which is effected in us by Christ's passion, i.e. grace; and a prognostic, that is, a foretelling of future glory.
[St. Thomas Aquinas, Summa Theologiae 3.60.3.]