The term Sacrament refers to a particular celebration of Christ's salvific work.  They have a visible and invisible reality, a reality open to all the human senses but grasped in its God-given depths with the eyes of faith.    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 water for Baptism.  The invisible reality, we cannot "see", is God's grace, his gracious initiative in redeeming us through the death and Resurrection of his Son.  This grace 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.  

A Sacrament is an efficacious sign of grace, instituted by Christ, and entrusted to the Church, by which divine life is dispensed to us by the work of the Holy Spirit.
-CC, nos. 1131, 774