Concubinatus (Latin, "concubinage") was a monogamous union, intended to be of some duration but not necessarily permanent, that was socially and to some extent legally recognized as an alternative to marriage in the Roman Empire. Concubinage became a legal concern in response to Augustan moral legislation that criminalized adultery and imposed penalties on some consensual sexual behaviors outside marriage.

Reasons for choosing concubinatus over marriage varied. If one partner was freeborn and belonged to the senatorial order, and the other was a former slave, there were legal penalties for marrying. More generally, wealthy widowers or divorced men might avoid the legal complexities of a second marriage in preserving their estates for heirs while still acknowledging a commitment to their partner. However, both partners might be freedpersons, with the benefits of concubinatus over marriage for people of this status not entirely clear in the historical record. Concubinatus was distinguished in Roman law from contubernium, a de facto marriage in which the partners were both slaves or one was a slave and the other a freedperson.

The female partner, "perhaps always" the person of lower social rank in the Classical period, was a concubina, literally a "bedmate", one for lying with, but a socially respectable role in contrast to the paelex, a sexual partner who was a rival to a wife. The naming of a concubina as such in epitaphs indicates that she was accepted as part of the extended family, and juristic texts accord the concubina certain protections. The listing of concubinae along with legal wives on grave markers indicates serial monogamy, not their coexistence, as tombs were often communal and included multiple members of a household, from different times of the male partner's life.

In Latin literature, however, concubinae are more often disparaged as female slaves kept as sexual luxuries, sometimes along with eunuchs. The discrepancy lies in whether the union was legally verifiable as monogamous concubinatus; an ancilla (female slave as part of a household) might be kept as a "bedmate" and referred to as concubina but was not eligible for the privileges of formal concubinage. The equivalent term for a male, concubinus, is used only informally, most often for a same-sex relationship.

Although usage of the word concubina during the Roman Empire poses ambiguities of role and status, the difference between the Imperial-era concubine as a subject of legal interest and a paelex or extralegal concubine during the Republic is fairly straightforward: the paelex was a woman "installed" by a married man as a sexual rival to his wife, whereas the concubina was a wife-like companion as well as sexual partner.

According to the 2nd-century antiquarian Aulus Gellius, in early Rome paelex was a disparaging word for a woman in a continuing sexual liaison with a man who had also contracted an archaic form of marriage cum manu, meaning that he held patriarchal power over his wife. In a much-cited law attributed to the semilegendary Numa Pompilius, second king of Rome (ca. 716–673 BCE), a concubine (paelex, not a concubina) was barred from the cultivation of Juno, the goddess of marriage: "a paelex shall not touch the altar of Juno. If she touches it, she shall sacrifice, with her hair unbound, a ewe lamb to Juno." The paelex in Latin literature is a woman perceived as a sexual threat by the wife, just as Juno was perpetually aggrieved by her husband Jupiter's "affairs", few if any of which were perpetrated with willing partners. In mythology, particularly in the cycle of myths pertaining to the Trojan War, the paelex is often a war captive and hence slave brought into the home as booty by the returning husband. The word paelex is so used by the comic playwright Plautus and was the title of a lost play by Naevius. Writing under Augustus, Ovid often uses the word paelex for abducted or captive women and for non-wives subjected to domestic rape in the myths he depicts in the Metamorphoses and other works. In these stories, the paelex is often depicted as foreign or barbarian.

As concubinatus became regularized under Imperial law and the status of the concubina elevated as the extralegal equivalent of a wife within imposed monogamy, usage of paelex inversely degraded so that it came to mean nothing more than a woman who had sex with a married man, and in late antiquity seems to have been a synonym for "prostitute" or "whore".

During the reign of Augustus, the first Roman emperor, certain forms of sexual conduct outside marriage, including some consensual behaviors, were criminalized as stuprum, illicit sexual intercourse, sometimes translated as "criminal debauchery" or "sex crime". Stuprum encompasses diverse sexual offenses including rape and adultery. Stuprum could only be committed against a citizen in good standing; slaves and prostitutes were neither protected by nor liable to these laws, nor were the infames, those whose social standing was permanently compromised by their professions or offenses against public morality.