Modern Catholic Dictionary:
CHASTITY. The virtue that moderates the desire for sexual pleasure according to the principles of faith and right reason. In married people, chastity moderates the desire in conformity with their state of life; in unmarried people who wish to marry, the desire is moderated by abstention until (or unless) they get married; in those who resolve not to marry, the desire is sacrificed entirely.
Chastity and purity, modesty and decency are comparable in that they have the basic meaning of freedom from whatever is lewd or salacious. Yet they also differ. Chastity implies an opposition to the immoral in the sense of lustful or licentious. It suggests refraining from all acts or thoughts that are not in accordance with the Church’s teaching about the use of one’s reproductive powers. It particularly stresses restraint and an avoidance of anything that might defile or make unclean the soul because the body has not been controlled in the exercise of its most imperious passion. (Etym. Latin castus, morally pure, unstained.)
CELIBACY. The state of being unmarried and, in Church usage, of one who has never been married. Catholicism distinguishes between lay and ecclesiastical celibacy, and in both cases a person freely chooses for religious reasons to remain celibate.
Lay celibacy was practiced already in the early Church. The men were called “the continent” (continentes) and women “virgins” (virgines). They were also known as ascetics who were encouraged to follow this form of life by St. Paul. According to the Apostle, “An unmarried man can devote himself to the Lord’s affairs, all he need worry about is pleasing the Lord . . . In the same way an unmarried woman, like a young girl, can devote herself to the Lord’s affairs; all she need worry about is being holy in body and spirit” (I Corinthians 7:32, 34). Throughout history the Church has fostered a celibate life in the lay state. Towering among the means of sanctity available to the laity, declared the Second Vatican Council, “is that precious gift of divine grace given to some by the Father to devote themselves to God alone more easily with an undivided heart in virginity or celibacy. This perfect continence for love of the kingdom of heaven has always been held in high esteem by the Church as a sign and stimulus of love, and as a singular source of spiritual fertility in the world” (Constitution on the Church, 42).
Ecclesiastical celibacy was a logical development of Christ’s teaching about continence (Matthew 19:10-12). The first beginnings of religious life were seen in the self-imposed practice of celibacy among men and women who wished to devote themselves to a lifetime following Christ in the practice of the evangelical counsels. Celibacy was one of the features of the earliest hermits and a requirement of the first monastic foundations under St. Pachomius (c. 290-346). Over the centuries religious celibacy has been the subject of the Church’s frequent legislation. The Second Vatican Council named chastity first among the evangelical counsels to be practiced by religious and said that “it is a special symbol of heavenly benefits, and for religious it is a most effective means of dedicating themselves wholeheartedly to the divine service and the works of the apostolate” (Decree on the Up-to-date Renewal of Religious Life, 12). (Etym. Latin caelibatus, single life, celibacy.)