n.1 a religious ceremony or act of the Christian Churches regarded as an outward and visible sign of inward and spiritual grace: applied by the Eastern, pre-Reformation Western, and Roman Catholic Churches to the seven rites of baptism, confirmation, the Eucharist, penance, extreme unction, ordination, and matrimony, but restricted by most Protestants to baptism and the Eucharist.
2 a thing of mysterious and sacred significance; a sacred influence, symbol, etc.
3 (also Blessed or Holy Sacrament) (prec. by the) a the Eucharist. b the consecrated elements, esp. the bread or Host.
4 an oath or solemn engagement taken.
Etymology: ME f. OF sacrement f. L sacramentum solemn oath etc. f. sacrare hallow f. sacer SACRED, used in Christian L as transl. of Gk musterion MYSTERY(1)