Definition
Voluntary sexual intercourse between a married person and someone other than that person's spouse. At minimum, one party to the act must be married for adultery to be committed. Most American jurisdictions historically treated it as both a criminal offense and a recognized ground for civil divorce.
Two gradations appear across historical sources:
(1) Single adultery: intercourse between a married person and an unmarried person.
(2) Double adultery: intercourse between two persons both of whom are married, though not to each other.
The distinction carried weight in ecclesiastical courts and in some American criminal statutes, though it rarely altered the basic elements of the offense.
Common Language
Modern common usage (Wiktionary): Sexual intercourse by a married person with someone other than their spouse. Wiktionary also records extended senses — lewdness of thought as a biblical category, faithlessness in religion, and "the fine and penalty formerly imposed for the offence" — none of which carry legal weight in modern doctrine.
Historical common usage (Webster's 1913): "The unfaithfulness of a married person to the marriage bed; sexual intercourse by a married man with another than his wife, or voluntary sexual intercourse by a married woman with another than her husband." Webster's also preserves the theological senses: unchastity of thought and faithlessness in religion.
The gap between common and legal meaning here is subtle but consequential. Ordinary usage treats adultery as bilateral — something both participants commit. Legal usage historically asked a sharper question: which party's marriage is the operative one? Some jurisdictions held that an unmarried participant could not be guilty of adultery as a principal, only as an accessory or under a separate fornication charge. Researchers working with criminal indictments or divorce pleadings must attend to this asymmetry.
Common Confusion
Adultery vs. Fornication: Fornication is sexual intercourse between two unmarried persons. At common law, adultery required that at least one party be married. An unmarried person who had intercourse with a married person was guilty of fornication in some jurisdictions, not adultery — a distinction that matters when reading indictments, ecclesiastical proceedings, or divorce records that itemize charges separately.
Adultery vs. Criminal Conversation: Criminal conversation (crim. con.) was a common law tort — a husband's civil action against a third party for sexual intercourse with his wife. The underlying act is the same, but criminal conversation is a civil remedy pursued against the paramour, while adultery is the criminal or matrimonial offense attributed to the offending spouse. The two terms appear in overlapping historical sources and are easily conflated.
Recognized Forms
/SUBTYPES
Single adultery: Intercourse between a married person and an unmarried person. Historically treated as less severe in ecclesiastical law.
Double adultery: Intercourse between two persons each of whom is married to a different spouse. Treated as the graver offense by Burrill and by ecclesiastical courts, sometimes warranting stricter censure.
Why It Matters in Research
Several traps await researchers working across time periods and source types.
Criminal versus matrimonial adultery. Historical sources use "adultery" to describe two distinct legal contexts: criminal prosecution (indictment, fine, imprisonment) and matrimonial cause (ground for divorce or separation). These are not the same proceeding, do not require identical proof, and were heard in different courts. In England, matrimonial adultery was a matter for the ecclesiastical courts through most of the nineteenth century; criminal adultery was a separate and rarely pursued matter in the temporal courts. American jurisdictions collapsed these into unified court systems at varying points, but older digests and reporters often segregate them.
The common law rule on unmarried parties. Anderson's Dictionary flags that at common law adultery could not be committed with a single woman — meaning the unmarried participant was not the adulterer. Researchers reading early American decisions must be alert to this rule, which affected who could be indicted, who was named as co-respondent in divorce suits, and what proof was required.
Shift from fault to no-fault divorce. Adultery's practical importance in modern American law lies almost entirely in divorce and its downstream consequences — property division, alimony, and occasionally child custody — rather than in criminal prosecution. The no-fault revolution beginning in the 1970s displaced adultery as a required element of divorce, but fault-based grounds (including adultery) remain operative in a significant number of states for equitable distribution and alimony calculations. A researcher moving between pre- and post-no-fault materials will find that the term carries very different weight depending on the era and the relief sought.
Gender asymmetry in historical sources. Pre-twentieth-century definitions frequently frame adultery in terms of the wife's infidelity as the primary offense — reflecting the common law's concern with legitimacy of issue and the husband's property interest in the marital relationship. This asymmetry is visible in Anderson's definition, which leads with "a married woman." Researchers should not assume that historical definitions apply symmetrically across gender.
Corpus connections. Adultery appears throughout divorce pleadings, ecclesiastical court records, criminal indictments, and treatises on domestic relations. When working in the Law Mind corpus, connecting this entry to the encyclopedia entry on fault-based divorce grounds (family_39) will surface the procedural and evidentiary standards that governed adultery as a matrimonial cause.
Historical Dictionary Support
Black's (both editions) and Bouvier's largely agree: adultery is voluntary sexual intercourse by a married person with someone not their spouse, and both parties are guilty when only one is married. This formulation reflects the majority American rule by the late nineteenth century.
Burrill adds the single/double adultery distinction and provides useful historical context: in England, adultery was anciently punished criminally but had by Burrill's time retreated entirely to the ecclesiastical courts, with temporal courts reaching it only indirectly (as through criminal conversation). This jurisdictional split is essential for researchers working with English legal materials.
Anderson's preserves the common law rule that adultery cannot be committed with a single woman — a point Black's and Bouvier's soften or omit, though their formulations are not irreconcilable with it.
Rapalje & Lawrence's surviving excerpt addresses ecclesiastical censure (suspension) rather than the substantive definition, which limits its utility here but confirms the ecclesiastical law dimension.
What the historical dictionaries collectively understate: the procedural bifurcation between criminal and matrimonial adultery, and the degree to which proof standards differed between them. Researchers relying solely on these definitions may miss that a divorce court and a criminal court would assess the same underlying conduct under different rules.
Jurisdictional Note
Adultery remains a criminal offense on the books in a number of American states, though prosecution is rare and some statutes have been struck down or fallen into desuetude. Its greater practical relevance today is in equitable distribution and alimony determinations in states that retain fault as a statutory factor. English law decriminalized adultery as a matrimonial offense concept through the shift to no-fault divorce under the Matrimonial Causes Act 1973, though the term persists in older materials.
Encyclopedia Cross-Reference
Divorce — Fault-Based Grounds (Adultery, Cruelty, Desertion, Imprisonment), The Law Mind Family Law Encyclopedia (family_39)