An ordinance is a Christian rite, associated with tangible elements (water; bread and wine), that is celebrated by the church of Jesus Christ. The term is closely associated with the word sacrament, which is an outward and visible sign of an inward and invisible grace.https://www.thegospelcoalition.org/essay/the-ordinances-of-the-church/
A sacrament is a holy ordinance instituted by Christ, in which by sensible signs the grace of God in Christ, and the benefits of the covenant of grace, are represented, sealed, and applied to believers, and these, in turn, give expression to their faith and allegiance to God.Systematic Theology, 527
According to the Reformed, the sacramental union is a spiritual bond, effected by God the Holy Spirit, and received by faith, so that by receiving the sign (bread, water, wine), the thing signified is also received (the promises of the covenant, the forgiveness of sins and participation in the resurrection life of Christ).http://www.christreformed.org/lecture-5a/
Remember that the ordinances of Christ are not His grave wherein he lies but the throne whereon He sits as king of His church; that you should never be above ordinances until you are above temptations; and that they lose not their authority because sometimes we miss their influence; and that they are appointed not only to bring men to but also to build men up in Christ.
The ordinances. These are the conduits Jesus Christ hath instituted and appointed His ordinances to be the means of carrying His nourishing virtue to the soul. The ordinances are the dishes of gold upon which this heavenly meat is brought. Prayer, reading, preaching, meditation, holy conference, the sacrament—in these Christ presents Himself to the soul. He that forsakes these can expect no feeding from Christ.
Ordinances—they are the golden ladder by which the soul climbs up to heaven; they are conduits of the water of life. O how precious should these be to us! They that are against ordinances are against being saved.
According to the Scriptures, sacraments contain an outward or visible element. That is, sacraments are based upon material objects: water in baptism, bread and wine in the Supper. But a sacrament not only includes the material element prescribed in Scripture, but this also extends to the rite itself as commanded by Scripture. As we have seen, this language is used in connection with the Covenant made with Noah (Genesis 9:12-13), the covenant made with Abraham (Genesis 17:11); and as confirmed by Paul (Romans 4:11).http://www.christreformed.org/lecture-5a/
The church is distinguished and contained by the right administration of baptism and the Lord's Supper. It should also be noted that this latter mark presumes and implies the practice of church discipline.The Church: The Gospel Made Visible (Kindle Locations 745-746). B&H Publishing Group. Kindle Edition.