Fornication or adultery? Two words that are often confused

How do dictionaries define fornication and adultery?

Dictionaries define fornication as consensual sex between people who are not married to each other, and adultery as sex involving a person who is married to someone else. Merriam-Webster's entry for fornication spells out the contrast in a usage note: in legal use, adultery applies only when at least one party is married, whereas fornication can describe two unmarried people, whether they are unmarried to each other or to others ^[1]. The marital status of the people involved is the whole difference.

That test produces three simple cases. If neither person is married, the conduct is fornication. If one person is married and the other is not, it is adultery for the married person and, depending on the usage, also for the unmarried partner. If both are married, but not to each other, it is adultery on both sides. What the dictionary does not do is rank them: the definitions are descriptive, and the moral weight attached to each word comes from religion and law rather than from the dictionary.

The same entry carries one further warning that is useful in everyday writing. Merriam-Webster notes that "adultery" and "adult" share their first five letters but are not related in origin ^[1]. Many people assume that adultery is simply what adults do, but the word has nothing to do with adulthood, as the next section explains.

Both words are formal. A learner's dictionary labels fornication as formal and disapproving, as the page on the meaning and origin of fornication records. In ordinary British speech, people say "having an affair" for adultery and "sleeping together" or "sex before marriage" for the other, which tells you how far the two Latin-derived words have moved into specialist registers.

Fornication and adultery compared

Point of comparison	Fornication	Adultery
Dictionary meaning	Consensual sex between two people not married to each other [1]	Sex between a married person and someone other than their spouse [1]
First recorded in English	c. 1300, from Old French fornicacion [3]	c. 1300, from Old French avouterie [2]
Latin source	fornicari, from fornix (arch, brothel) [3]	adulterare, 'to corrupt', from ad + alterare [2]
King James Bible, Galatians 5:19	Listed as a work of the flesh [4]	Listed first, just before fornication [4]
Act of 10 May 1650	Three months in gaol, then a year's security for good behaviour [7]	Death for a married woman and, unless he did not know she was married, her partner [7]
Divorce law in England and Wales	Never a ground for divorce	A fact proving breakdown until 6 April 2022; since then no facts are needed [5][10]
Divorce law in Scotland	Never a ground for divorce	Still one of the facts proving irretrievable breakdown [9]

Where do the two words come from?

Both words came into English around 1300 from Old French, and both ultimately come from Latin, but from different roots. Etymonline dates fornication to about 1300, from Old French fornicacion, and adultery to about 1300, from Old French avouterie, a form recorded in the twelfth century ^[2][3]. The two arrived together because English at that time was absorbing the French vocabulary of law and religion wholesale.

The Latin sources differ. Fornication descends from fornicari, a verb built on fornix, a Latin word for an arch that Roman poets also used for a brothel ^[3]. Adultery descends from Latin adulterare, meaning to commit adultery or to corrupt, which combines ad ("to") and alterare ("to alter") ^[2]. A related Etymonline entry, on adulteration, cites an alternative reading by the linguist Calvert Watkins, who takes the word to mean "(approaching) another (unlawfully)", from ad alterum ^[2].

English had a native word before the French one arrived. Etymonline records an Old English word for adultery, aewbryce, meaning a breach of lawful marriage ^[2]. English speakers therefore had a native word for breaking a marriage before the French term arrived.

The family of words around adultery is large. Etymonline lists adulteration (about 1500) among the relatives of the word, and alter, the Latin word contained in the verb, which is why a food that has been "adulterated" has literally been altered or corrupted ^[2]. The agent noun "adulterer" appears in the early fifteenth century, formed from an obsolete verb "adulter", and "adulterous" around 1600 ^[12]. Spelling was also adjusted towards Latin in the early fifteenth century, which is why the modern form is so different from the French avouterie ^[2].

How does the King James Bible use them?

The King James Bible of 1611 uses both words and often lists them next to each other, treating them as separate things. Galatians 5:19 gives "Adultery, fornication, uncleanness, lasciviousness" as the first items in its list of the works of the flesh ^[4]. In 1 Corinthians 6:9 the same translators write that "neither fornicators, nor idolaters, nor adulterers" will inherit the kingdom of God ^[6].

The pairing in these lists shows that the translators understood two distinct categories. Adultery is the breaking of an existing marriage; fornication is wider, covering sexual conduct outside marriage generally. Etymonline adds that the Bible extended the strict meaning of fornication, a point that explains why the word can also be found in contexts where it means sexual unfaithfulness in a broad sense ^[3].

The Church of England's liturgy worked in the same vocabulary. The marriage service in the Book of Common Prayer of 1662 gives three reasons for which matrimony was ordained, one of them as a remedy against sin and to avoid fornication, for those who lack the gift of continency ^[13]. The word "adultery" does not appear in that formula, because the concern there is the state of people who are not yet married.

A caution applies to this kind of word study. English Bible translations differ, and the Greek and Hebrew words behind "fornication" and "adultery" have wider or narrower ranges than the English words. The King James Version is used here because it fixed the English forms that most British readers know, not because it settles what the original texts meant. Different Christian traditions read the lists differently, and this encyclopedia does not rank those readings.

How did English law treat each one?

English law treated adultery more seriously than fornication from the seventeenth century onwards, although in practice both were left to church courts for most of the period. The clearest early statutory contrast is the Act of 10 May 1650, titled an Act for suppressing the detestable sins of incest, adultery and fornication. It made adultery by a married woman a felony punishable by death, with the man also liable unless he did not know she was married. A man who had sex with an unmarried woman or a widow, and the woman herself, fell under the fornication clause instead, which prescribed three months in gaol and then a year's security for good behaviour ^[7]. The Act took effect for offences committed from 24 June 1650 ^[7].

That Act did not survive the Restoration. William Blackstone, writing in the 1760s, observed that its penalties were not revived afterwards and that the offences went back to the spiritual courts under canon law ^[11]. He added that the temporal courts took notice of adultery only as a private injury, meaning that a husband could sue the other man for damages, a claim known as criminal conversation ^[11]. Incontinence in general was left to what he called the feeble coercion of the church courts ^[11], a theme developed in the article on Blackstone and fornication.

The nineteenth century changed adultery from a private wrong into a ground for ending a marriage. The Matrimonial Causes Act 1857, which received royal assent on 28 August 1857 and took effect on 1 January 1858, moved divorce from the church courts to a new civil Court for Divorce and Matrimonial Causes ^[8][14], and its arrival is described as the end of the action for criminal conversation ^[15]. A husband could petition on his wife's adultery alone, but a wife had to prove adultery together with another offence such as cruelty or desertion, a difference that lasted until the Matrimonial Causes Act 1923 ^[8].

Fornication never became a ground for divorce, because it cannot occur between spouses. In England and Wales, adultery was one of the facts on which a spouse could rely, alongside unreasonable behaviour and separation, to show that a marriage had broken down irretrievably. The ONS divorce bulletin for 2022 shows how the picture later shifted: unreasonable behaviour accounted for 47.1% of opposite-sex petitions filed under the old law ^[10]. The broader history of the subject is told in the article on adultery and divorce law in England.

Do the words still matter legally in the UK?

Neither word is a crime in the United Kingdom today, but adultery still has a role in divorce law in Scotland, and its role in England and Wales has recently disappeared. Section 1 of the Matrimonial Causes Act 1973 was rewritten by the Divorce, Dissolution and Separation Act 2020: either or both spouses can now apply on the single ground that the marriage has broken down irretrievably, and the court must treat the statement as conclusive ^[5]. The Divorce, Dissolution and Separation Act 2020 received Royal Assent on 25 June 2020, and its main provisions took effect on 6 April 2022 under SI 2022/283 ^[5].

The practical result is that nobody has to allege adultery, or any other fault, to divorce in England and Wales. The Office for National Statistics reported 80,057 divorces in 2022, the lowest number since 1971, of which 7,394 (9.2%) had been granted under the new law, and 28.8% of those were joint applications ^[10]. The court cannot make a conditional order until 20 weeks after proceedings start, and the order cannot be made final until six weeks after that ^[5].

Scotland kept the older structure. Section 1(2)(a) of the Divorce (Scotland) Act 1976 still lists adultery by the defender after the marriage as one way of proving irretrievable breakdown, with exceptions where the pursuer connived at it or condoned it by continuing to live together knowing of it ^[9]. Other facts include unreasonable behaviour and periods of separation of one or two years ^[9]. Adultery has the same meaning for same-sex and opposite-sex marriages in that Act ^[9]. Northern Ireland also still allows fault-based divorce, though the details lie outside this page.

For fornication, the legal position is simpler. Because there is no offence of sex between consenting unmarried adults in any part of the UK, the word has no legal function now. It survives in the language of religion and history, which is why the subject of this encyclopedia is the word and the ideas around it rather than a rule in force. The question of whether any rule remains is answered in the article on whether fornication is illegal in the UK.

Frequently asked questions

Can a married person commit fornication?

Strictly, no, because the dictionary definition is sex between two people who are not married to each other. A married person who has sex with someone other than their spouse is committing adultery. In religious writing, however, "fornication" is sometimes used loosely for any sexual conduct outside marriage, so you will see it applied more widely ^[1][3].

Is adultery still a ground for divorce in the UK?

It depends on where you live. In England and Wales, adultery has not been needed since 6 April 2022, when a statement of irretrievable breakdown became enough ^[5]. In Scotland, adultery remains one of the facts that can prove irretrievable breakdown under the Divorce (Scotland) Act 1976 ^[9].

Was adultery ever a crime in England?

Yes, briefly. The Act of 10 May 1650 made adultery by a married woman a capital felony, but it was not revived after the Restoration ^[7][11]. After that, English temporal courts treated adultery only as a private injury to the husband, and the church courts dealt with it as a sin ^[11].

Does the King James Bible treat the two as different sins?

Yes, it lists them separately. Galatians 5:19 names adultery and fornication one after the other among the works of the flesh, and 1 Corinthians 6:9 names fornicators and adulterers as distinct groups ^[4][6]. How each tradition interprets the distinction varies, and this page reports the wording, not a doctrine.

Are 'adultery' and 'adult' related words?

No. Adultery comes from Latin adulterare, to corrupt, formed from ad and alterare, while adult comes from a different Latin root. Merriam-Webster points out that the two words merely look alike ^[1][2]. The resemblance is a coincidence of spelling.

Was there a difference between how the law treated husbands and wives?

Yes, for a long time. Under the Matrimonial Causes Act 1857, a husband could divorce his wife for adultery alone, but a wife had to prove adultery together with another offence such as cruelty or desertion ^[8]. The rule was equalised by the Matrimonial Causes Act 1923 ^[8].