Both are religious and legal terms for sex outside marriage, distinguished by whether anyone involved is married. Adultery involves at least one married person. Fornication is sex between two unmarried people. The words are archaic in everyday English and survive mainly in religious and legal contexts.
The core difference
- adultery — sex where at least one party is married to somebody else.
- fornication — sex between people neither of whom is married.
Marriage is the whole distinction
In the traditions that use these terms, adultery is the graver offence because it breaks an existing covenant — a third party is wronged. Fornication concerns the act itself rather than a broken commitment. This is why adultery appears in the Ten Commandments and fornication does not.
The legal position
Adultery retains legal significance in some jurisdictions, historically as grounds for divorce and in a few places as a criminal offence. In England and Wales it was a fact by which irretrievable breakdown could be shown until no-fault divorce was introduced in 2022; it also had a narrow definition that limited which acts qualified.
Fornication statutes existed in several US states and have largely been repealed or rendered unenforceable.
Where the words come from
Adultery does not come from adult. It is from Latin adulterare, to corrupt or debase — the same root that gives adulterate, meaning to contaminate a substance. The image is of a marriage being polluted by something foreign. The resemblance to adult, from adolescere, to grow up, is coincidence.
Fornication has an unexpectedly concrete origin: Latin fornix, an arch or vault. Prostitutes in ancient Rome worked in the vaulted arches beneath buildings, so fornicari came to mean visiting those arches. The architectural sense survives in anatomy, where the fornix is an arched bundle of fibres in the brain.
Register
Both words now sound distinctly archaic or homiletic in general English. Ordinary usage has affair, cheating or infidelity for the first, and no real equivalent for the second, because the concept has largely ceased to be one English needs a word for.
Frequently asked questions
What is the difference between adultery and fornication?
Adultery involves at least one married person. Fornication is sex between two unmarried people. Marriage is the whole distinction.
Does adultery come from the word adult?
No. It comes from Latin adulterare, to corrupt or debase — the same root as adulterate. The similarity to adult is coincidence.
Where does fornication come from?
From Latin fornix, an arch or vault. Prostitutes in Rome worked in vaulted arches beneath buildings, and the word followed from that.