Although the definition of "adultery" seems to differ in nearly every legal system, the common theme is sexual relations outside of marriage, in one form or another.
For example, New York defines adultery as a person who "engages in sexual intercourse with another person at a time when [they have] a living spouse, or the other person has a living spouse." While in North Carolina adultery is when any a man and woman who "lewdly and lasciviously associate, bed and cohabit together." Minnesota defines adultery as: "when a married woman has sexual intercourse with a man other than her husband, whether married or not, both are guilty of adultery".
Adultery was known in earlier times by the legalistic term "criminal conversation" (another term, "alienation of affection", is used when one spouse deserts the other for a third person). The term originates not from adult, which is from Latin a-dolescere, to grow up, mature, a combination of a, "to", dolere, "work", and the processing combound sc), but from the Latin ad-ulterare (to commit adultery, adulterate/falsify, a combination of ad, "at", and ulter, "above", "beyond", "opposite", meaning "on the other side of the bond of marriage").
A marriage in which both spouses agree that it is acceptable for the husband or wife to have sexual relationships with other people other than their spouse is sometimes termed as open marriage. The resulting sexual relationships the husband or wife has with other people, although could be considered to be adultery in some legal jurisdictions, are not treated as such by the spouses.
Some cultures have a distinguished interpretation of the term infidelity: in some legal systems, it might be tolerated by the jurisdiction, while adultery is a crime. Still another matter is the reaction of the betrayed; if, on the other hand, he gives his consent, in the case of adultery the is called a cuckold, but in the case of infidelity a wittol (now an archaic term). On the other hand, infidelity is not only a sexual term, but is the Latin word "unfaithful" (fides: faith). Having no faith can also mean the religious belief.NATURE OF ADULTERY ACCORDING TO CATHOLICS
Adultery is defined as carnal connection between a married person and one unmarried, or between a married person and the spouse of another. It is seen to differ from fornication in that it supposes the marriage of one or both of the agents. Nor is it necessary that this marriage be already consummated; it need only be what theologians call matrimonium ratum. Sexual commerce with one engaged to another does not, it is most generally held, constitute adultery. Again, adultery, as the definition declares, is committed in carnal intercourse. Nevertheless immodest actions indulged in between a married person and another not the lawful spouse, while not of the same degree of guilt, are of the same character of malice as adultery (Sanchez, De Mat., L. IX. Disp. XLVI, n. 17). It must be added, however, that St. Alphonsus Liguori, with most theologians, declares that even between lawful man and wife adultery is committed when their intercourse takes the form of sodomy (S. Liguori L. III, n. 446).