Fornication or Adultery? How the Two Words Differ

How do dictionaries distinguish fornication from adultery?

Dictionaries separate the two words by a single test: whether either partner is married. Merriam-Webster defines adultery as voluntary sexual intercourse between a married person and someone other than that person's current spouse, and fornication as consensual intercourse between two people who are not married to each other ^[1][2]. Its usage note makes the rule explicit: in legal use, adultery applies only when at least one participant is married, while fornication can describe consensual sex between unmarried people ^[1].

Cornell Law School's Wex dictionary follows the same division. It defines fornication as intercourse between two unmarried individuals, a conduct now more commonly called premarital sex ^[3]. Adultery, in Wex, is extramarital intercourse between a married person and someone who is not his or her spouse, a definition supported by a 2020 court decision, Gerges v. Gerges ^[4].

The two words also have different histories in English. Merriam-Webster dates fornication to the 14th century and the modern form adultery to the 15th ^[1][2]. Adultery comes from the Latin verb adulterare, "to pollute, defile, commit adultery," built from ad- and alter, "other"; despite appearances, it has no link with adult, which comes from a Latin participle meaning grown up ^[2].

The definitions leave one practical consequence. When a married person and an unmarried person have sex, dictionaries call the act adultery, because one participant is married. Whether the unmarried partner is also an adulterer depends on the text applied, and state laws have answered that question in different ways.

Fornication and adultery side by side

Source	Fornication	Adultery
Merriam-Webster	Consensual intercourse between two people not married to each other	Voluntary intercourse between a married person and someone other than the current spouse
South Carolina Code §§ 16-15-70, 16-15-80	Cohabitation or habitual intercourse, both partners unmarried	Same conduct when either partner is married to someone else; same penalty (§ 16-15-60)
Minnesota statutes (repealed in 2023)	Any man and a single woman	A married woman with any man other than her husband; both guilty
Michigan, MCL 750.30 (1931)	No offense in this section	Felony; an unmarried man with a married woman is also guilty
New Testament Greek (KJV)	porneia, 26 occurrences, all rendered fornication	moicheia, 4 occurrences, all rendered adultery
Catechism of the Catholic Church	§ 2353: union of an unmarried man and an unmarried woman	§ 2380: marital infidelity; § 2381 calls it an injustice

How do state laws define each offense?

State codes that punish both offenses usually reproduce the dictionary line, with local variations. South Carolina's is the clearest example. Section 16-15-70 defines adultery as living together and carnal intercourse, or habitual intercourse without living together, between a man and a woman when either is lawfully married to someone else; section 16-15-80 defines fornication as the same conduct when both are unmarried ^[5].

The two offenses share one penalty clause. Under section 16-15-60, a person guilty of either can be fined $100 to $500, imprisoned for six months to a year, or both, and all three sections trace back to an act of 1880 ^[5]. In South Carolina, then, the unmarried partner of a married person commits adultery, not fornication.

Other states define each offense on its own terms. Georgia's fornication statute, in the 2025 code, applies to an unmarried person who voluntarily has intercourse with another person, and looks only at the status of the person charged ^[6]. Illinois goes further: its fornication offense covers anyone who knowingly has intercourse with a person other than his or her spouse, provided the behavior is open and notorious, which means a married person can commit fornication there ^[7]. The same section exempts a person whose liability rests solely on evidence given to comply with the state's Public Aid Code ^[7].

Michigan is the strictest on adultery. Its penal code makes adultery a felony, a provision dating from Act 328 of 1931 and still printed in the compiled laws updated through 2026 ^[10]. New York, by contrast, repealed its adultery law in November 2024; the 1907 statute had made it a Class B misdemeanor, punishable by up to three months in jail ^[11].

Why did some laws depend on the woman's marital status?

Some older statutes defined both offenses by the woman's status rather than by the couple's. Minnesota offers the clearest case. As quoted by the state House in 2022, its adultery statute applied when a married woman had intercourse with a man other than her husband, whether he was married or not, and made both guilty ^[8]. Its fornication statute applied when any man and a single woman had intercourse ^[8].

Under those texts, the same conduct received different names depending on which partner was married. A married woman with an unmarried man committed adultery, and so did he. A married man with an unmarried woman fell under the fornication statute instead, because the woman was single ^[8]. Minnesota's code now shows the adultery section repealed by the 2023 session laws ^[9].

Michigan's 1931 statute keeps a trace of the same concern. It specifies that when the offense is committed between a married woman and an unmarried man, the man is guilty of adultery and liable to the same punishment ^[10]. Merriam-Webster's legal definition of fornication echoes the tradition by describing intercourse between a man and especially a single woman ^[1].

The asymmetry has ancient precedents. In the Hebrew Bible, the verb naʾaph, "to commit adultery," occurs 31 times, and the Brown-Driver-Briggs lexicon notes that it always concerns the wife of another man ^[12]. The commandment of Exodus 20:14 uses this verb, and Leviticus 20:10, which deals with a man and another man's wife, prescribes death for both participants ^[12]. None of the American statutes cited here states its reasons, so any link between these texts remains a matter of interpretation.

How does the Bible use the two words?

The New Testament has two distinct Greek words, and the King James Version renders them consistently. Moicheia appears four times in the Textus Receptus, the Greek text behind the KJV, and is translated as adultery each time ^[13]. Porneia appears 26 times and is always rendered fornication ^[14].

The two words often stand side by side, which shows that the writers distinguished them. In Matthew 15:19 the KJV lists "adulteries, fornications" among the evils that come from the heart, and Galatians 5:19 opens its list of the works of the flesh with adultery and fornication ^[13]. Thayer's lexicon cites these verses, with Mark 7:21, as places where porneia is kept distinct from adultery ^[14].

Elsewhere porneia covers more ground. Thayer's lexicon notes that it refers to adultery in Matthew 5:32 and 19:9, the passages on divorce ^[14]. Etymonline makes the same general point about English: the strict sense of fornication concerned two unmarried people, but the Bible extended the word to adultery ^[18]. The overlap explains why readers of the KJV sometimes find fornication applied to a married woman.

Both vocabularies also have a figurative side. Porneia can metaphorically mean the worship of idols ^[14], and the Hebrew verb for adultery is used in the same way: Brown-Driver-Briggs lists it for idolatrous worship, and in Jeremiah 3:9 Israel is said to commit adultery with stones ^[12]. In those passages neither word describes a sexual act at all.

Modern translators disagree on how to render Matthew 5:32. The KJV keeps "saving for the cause of fornication," the New International Version says "except for sexual immorality," the New Revised Standard Version "except on the ground of unchastity," the New Living Translation "unless she has been unfaithful," and the Catholic New American Bible "unless the marriage is unlawful" ^[15]. The debate behind these choices is explained in our reading of Matthew 5:32.

Which is treated more seriously?

In law and in church teaching, adultery has generally carried the heavier weight, though not everywhere. South Carolina punishes the two offenses identically ^[5]. Michigan, however, makes adultery a felony ^[10], while Georgia treats fornication as a misdemeanor ^[6], and Illinois classifies its narrow fornication offense as a Class B misdemeanor ^[7]. The ranking was never uniform: New York's former adultery law was itself only a Class B misdemeanor ^[11].

Civil law adds a further difference. Wex notes that adultery is a ground for divorce where fault-based divorce exists, and that some states let courts consider it when dividing property, awarding alimony or deciding custody ^[4]. Fornication involves no spouse and therefore plays no comparable role in family law.

Catholic teaching treats both as serious faults but describes them differently. The Catechism defines fornication in paragraph 2353, under the heading on offenses against chastity, as the union of an unmarried man and an unmarried woman, and calls it gravely contrary to the dignity of persons and of human sexuality ^[16]. Adultery appears later, in paragraph 2380, as marital infidelity ^[17].

Paragraph 2381 explains why adultery is graver in that framework: the Catechism calls it an injustice, because the adulterer breaks a vow, violates the rights of the other spouse and undermines the institution of marriage ^[17]. The added element is harm to a third person, the spouse who is not present.

Does the distinction still matter today?

The distinction still matters in three places: divorce law, the few criminal statutes that remain, and religious teaching. For fornication, Wex explains that the remaining state laws generally cannot be enforced since Lawrence v. Texas (2003), which protects private sexual conduct between adults ^[3]. No ruling striking down an adultery statute on that ground was identified for this article, so the status of the remaining adultery laws is less clear-cut.

Repeals continue all the same. New York's was signed in late 2024 after the bill's author, Assemblyman Charles Lavine, said that by his count 13 people had been charged under the 1907 law and five convicted, the most recent case, in 2010, having been dismissed ^[11]. Minnesota's code shows its former adultery section, 609.36, repealed in 2023 ^[9], while Michigan's felony remains in the compiled laws ^[10].

In everyday speech the line has blurred. Infidelity between unmarried partners is often described as "cheating," but dictionaries keep adultery for situations involving a marriage ^[2]. For the general meaning of the other word, see what fornication means, and for South Carolina's statutes in detail, our article on the state's adultery and fornication laws.

Frequently asked questions

Is adultery still a crime in some states?

Yes. Michigan's penal code still makes adultery a felony ^[10], and South Carolina punishes it with a fine of $100 to $500, six months to a year in prison, or both ^[5]. New York repealed its 1907 adultery law in November 2024 ^[11].

Can a single person commit adultery?

Yes, under several legal definitions. In South Carolina, adultery exists when either partner is married to someone else, so both partners commit it ^[5]. Michigan's statute states that an unmarried man who has intercourse with a married woman is guilty of adultery ^[10].

Does the Bible treat fornication and adultery as the same sin?

Not exactly. The New Testament uses two words, porneia and moicheia, and lists them separately in verses such as Matthew 15:19 and Galatians 5:19 ^[13]. Porneia is broader, however, and Thayer's lexicon notes that it refers to adultery in Matthew 5:32 and 19:9 ^[14].

Is adultery grounds for divorce in the US?

Yes, in states that allow fault-based divorce. According to Wex, some states also let courts weigh adultery when dividing property, setting alimony or deciding custody ^[4]. Fornication has no such role, since no spouse is involved.

Are the words adult and adultery related?

No. Adultery comes from Latin adulterare, "to pollute, defile," formed from ad- and alter, "other," while adult comes from a Latin participle meaning grown up ^[2]. The resemblance between the two English words is accidental.

Is cheating on a boyfriend or girlfriend adultery?

No, not in the dictionary or legal sense, because adultery requires that at least one participant be married ^[2]. Infidelity between unmarried partners has no special legal name. In strict religious vocabulary, sex between two unmarried people is called fornication ^[16].

Why did Minnesota's old law treat married men and women differently?

Because its definitions turned on the woman's status. Adultery applied when a married woman had intercourse with any man other than her husband, while a married man with a single woman fell under the fornication statute ^[8]. Minnesota's code shows the adultery section repealed in 2023 ^[9].