noun

• (Christianity) A sacred act and the attendant ceremony, considered (theology) an outward sign of divine grace, instituted by Jesus Christ.
  "Priest: I'm sorry, it's Duncan Dirk Dick. I've just done it. / Father: Well, undo it. / Priest: Undo it? / Father: Yes. / Priest: This is a holy sacrament of the Church, not a bleeding hotel reservation. You can't just undo it."
• (in particular) The Eucharist.
• (by extension) Anything regarded as possessing a sacred character or mysterious significance.
  "God ſometimes ſent a light of fire, and pillar of a cloud, and the brightneſſe of an Angel, and the luſtre of a ſtar, and the ſacrament of a rainbovve to guide his people thorovv their portion of ſorrovvs, and to lead them through troubles to reſt: […]"
• (Ancient Rome) The oath of allegiance taken by soldiers in Ancient Rome; hence, any sacred ceremony used to impress an obligation; a solemn oath-taking; an oath.
  "I'll take the sacrament on 't."

verb

• (transitive) To bind by an oath.