Chastity, also known as purity, is a virtue related to temperance. Someone who is chaste refrains from sexual activity that is considered immoral or from any sexual activity, according to their state of life. In some contexts, for example when making a vow of chastity, chastity means celibacy.

The words chaste and chastity stem from the Latin adjective castus ("cut off", "separated", "pure"). The words entered the English language around the middle of the 13th century. Chaste meant "virtuous", "pure from unlawful sexual intercourse" or (from the early 14th century on) as a noun, a virgin, while chastity meant "(sexual) purity".

Thomas Aquinas links castus (chastity) to the Latin verb castigo ("chastise, reprimand, correct"), with a reference to Aristotle's Nicomachean Ethics: "Chastity takes its name from the fact that reason 'chastises' concupiscence, which, like a child, needs curbing, as the Philosopher states".

For many Orthodox Jews, Christians, Muslims, and Bahá’í people should restrict their acts of a sexual nature to the context of marriage. For unmarried people, chastity is equivalent to sexual abstinence. Sexual acts outside of or apart from marriage, such as adultery, fornication, masturbation, and prostitution, are considered immoral due to lust.

In many Christian traditions, chastity is synonymous with purity. The Catholic Church teaches that chastity involves, in the words of cardinal bishop Alfonso López Trujillo, "the successful integration of sexuality within the person and thus the inner unity of man in his bodily and spiritual being", which according to one's marital status requires either having no sexual relationship, or only having sexual relations with one's spouse. In Western Christian morality, chastity is placed opposite the deadly sin of lust, and is classified as one of seven virtues. The moderation of sexual desires is also required to be virtuous. Reason, will, and desire can harmoniously work together to do what is good.

As an emblem of inward chastity, some Christians choose to wear a cord, girdle or a cincture of one of the several Confraternities of the Cord or a purity ring. The cord is worn as a symbol of chastity in honour of a chaste saint whom the bearer asks for intercession. The purity ring is worn before holy matrimony by those who marry or for the rest of their lives by those who stay single.

In marriage, the spouses commit to a lifelong relationship that excludes sexual intimacy with other persons. A third form of chastity, often called "vidual chastity", is expected by the society for a period after the woman's husband dies. For example, Anglican Bishop Jeremy Taylor defined five rules in Holy Living (1650), including abstaining from marrying "so long as she is with child by her former husband" and "within the year of mourning".

In the Roman Catholic Church, members of the consecrated life vow or promise celibacy as one of the evangelical counsels. In 306, the Synod of Elvira proscribed clergy from marrying. This was unevenly enforced until the Second Lateran Council in 1139 when it found its way into canon law. Unmarried deacons promise celibacy to their local bishop when ordained.