Look it up and it takes one line: a sexual act involving genital union. Yet the range this word covers has been rewritten again and again by era, law, and culture. Seikou (Japanese: 性交; English: sexual intercourse; Latin: coitus) is the general term for a sexual act involving the union of human genitals or bodies. Law, medicine, sociology, and sexology each hold their own definition. Narrowly it means vaginal intercourse (the union of vagina and penis); broadly it can include anal and oral intercourse (fellatio, cunnilingus). Japan’s current penal code adopts a definition of “intercourse, etc.” that bundles vaginal, anal, and oral intercourse, so the reach of the term differs greatly by field.

Overview

Seikou names a sexual act involving the union of human genitals or bodies, regardless of sex, body type, or relationship. In everyday speech and sexological writing it is used almost synonymously with “sex” (sekkusu), but in medicine, law, and statistics it is used under stricter definition.

In medicine and sexology, the narrow seikou means insertion of the male organ into the female organ; in the reproductive context this narrow sense was placed as the origin of fertility. With the development of sexology, the concept expanded into a comprehensive notion of sexual activity including oral and anal intercourse and masturbation, and modern sex education and sexology now also use the term in a broad sense. Intercourse is embedded in social institutions (marriage, family), religious norms, legal systems, and the entertainment industry in nearly every human society, an individual experience and at once a unit constituting culture.

Etymology

Seikou compounds sei (“sex, gender”) and kō (“to cross, to mingle”). Majiwaru (“to mingle”) has long denoted intimate relations between man and woman and conjugal union in classical Chinese and Japanese texts. Edo erotic and popular literature also circulated kōgō, kōsetsu, and chigiri synonymously. In the modern era, seikou was organised as a medical and legal term within the Meiji-Taishō system of medical translation, taking its place as the rendering of English sexual intercourse and Latin coitus.

The English intercourse derives from Latin intercursus (“a running between, exchange”) and originally denoted exchange broadly; from the fifteenth century it acquired sexual sense, and in modern English intercourse alone denotes intercourse. Coitus is the noun of Latin coire (“to go together, to unite”), current internationally as a medical term. In Japanese colloquial speech “sekkusu” (from English sex) is overwhelmingly common, while seikou is a comparatively formal, hard register used in academic, legal, and journalistic contexts.

Differences of definition by field

In law, “intercourse, etc.” (vaginal, anal, oral intercourse) is defined as an element of the offence of non-consensual intercourse (Penal Code Art. 177, revised 2023) and non-consensual indecency (Art. 176). The pre-revision offence (formerly rape) bundled penile insertion into the vagina, anus, or mouth as “intercourse, etc.”; the 2023 revision further widened the covered acts. In civil law, the concept of intercourse holds an indirect place (cohabitation and cooperation duties, judgement of marriage nullity, the premise of the presumption of legitimacy) but has no explicit defining provision.

In medicine and sexology, coitus traditionally meant the narrow insertion of the male organ into the female. The Kinsey reports (1948, 1953) and the Masters-and-Johnson report (Human Sexual Response, 1966) centred the narrow sense while presenting a comprehensive system of sexual activity including oral and anal intercourse and masturbation. Modern sexology, distinguishing the reproductive context from the pleasure-and-intimacy context, increasingly reads seikou broadly. The WHO’s definition of sexual health (2002) frames sexual activity, health, and rights as comprehensive concepts that include but are not limited to reproduction. In school sex education, intercourse was traditionally placed as “an act leading to pregnancy and birth,” taught from the angle of contraception and STI prevention; recently the framework of comprehensive sexuality education (CSE), centred on “consensual sexual relations” and “mutual respect,” has been recommended.

Structure of the act

Sexological analysis often discusses intercourse in a three-stage model: foreplay, coitus, and afterplay. Foreplay includes kissing, caressing, cunnilingus, and fellatio, raising both partners’ physical and psychological arousal. Coitus runs from penetration to the orgasm or ejaculation of one or both partners; Masters and Johnson’s sexual response cycle (1966) presents the four stages of excitement, plateau, orgasm, and resolution. When both partners’ orgasms occur close in time it is called simultaneous orgasm, long discussed in sexological writing as an “ideal state.” Afterplay includes post-coital contact, conversation, and embrace, placed as contributing to sustained closeness and the deepening of the relationship.

Vocabulary of contemporary Japanese

In everyday Japanese, words for intercourse are used by context. “Sekkusu” is the most common colloquial term, used widely across sex and age. “Seikou” is standard in formal legal, medical, and news contexts. “Seikōshō” (“sexual relations”) is a euphemism favoured in news. “Night life” and “the couple’s affairs” are euphemisms in the marital context. “Etchi” is a casual slang among the young. Verb usages such as “to do it” obscure the object to gain euphemism. In dōjinshi, adult manga, and AV genre names, “sekkusu” is nearly standard, while “seikou” is used to stage a classical or academic atmosphere.

Related terms

• Penetration (sounyuu)
• Creampie (nakadashi)
• Bareback (namahame)
• Orgasm (zecchou)
• Simultaneous orgasm (douji-zecchō)
• Missionary (seijoui)
• Cowgirl (kijoui)
• Rear-entry (back position)
• Fellatio (fera)
• Cunnilingus (kunni)
• Anal