The Westminster Larger Catechism

Answer: A sacrament is a holy ordinance instituted by Christ in his church, to signify, seal, and exhibit unto those that are within the covenant of grace, the benefits of his mediation; to strengthen and increase their faith, and all other graces; to oblige them to obedience; to testify and cherish their love and communion one with another; and to distinguish them from those that are without.