St. Augustine defined a sacrament as “the visible form of an invisible grace”.
The visible reality we see in the Sacraments is their outward expression, the form they take, and the way in which they are administered and received, such as the ceremony of a sacramental mass. The invisible reality we cannot "see" is God's grace, His gracious initiative in redeeming us through the death and Resurrection of His Son. His initiative is called grace because it is the free and loving gift by which He offers people a share in His life, and shows us His favor and will for our salvation. Our response to the grace of God's initiative is itself a grace or gift from God by which we can imitate Christ in our daily lives. - USCCB