Rape is a type of sexual assault initiated by one or more persons against another person without that person's consent. The act may be carried out by physical force, under threat or manipulation, by impersonation, or with a person who is incapable of giving valid consent.

Definitions of rape vary, but they generally require some degree of sexual penetration without consent. The term "consent" varies by law as well. Minors, for example, are often considered too young to consent to sexual relations with older persons (see statutory rape and age of consent). Consent is also considered invalid if obtained under duress, or from a person who does not have the ability to understand the nature of the act, due to factors such as young age, mental disability, or substance intoxication.

Many jurisdictions, such as Canada and several US and Australian states, no longer have a traditional common law offence of rape, which always required that sexual penetration had occurred. Some of these jurisdictions instead have created new statutory offences, such as sexual assault or criminal sexual conduct, which criminalise sexual contact without consent, and without any requirement that sexual intercourse occurred.

Depending on the jurisdiction, rape may be characterized as a sexual offence or an offence against the person. Rape may also be characterized as a form of aggravated assault or battery, or both, indecent assault or sexual assault or battery, or both.

To sustain a conviction, rape might require proof that the defendant had sexual penetration with another person. Depending on the jurisdiction, the actus reus of rape may consist of "having carnal knowledge of" a woman, or "having sexual intercourse with" a woman (including a girl) specifically, or either a woman or a man (including a girl or a boy) generally, or engaging in sexual intercourse with a person (which term includes an intersex person who might arguably be neither a woman nor a man) or having "sexual connection" with a person affected by penile penetration of that person's genitalia, or penile penetration of the vagina, anus or mouth (these terms construed as including surgically constructed organs) of a person.

In Prosecutor v. Anto Furundžija, the International Criminal Tribunal for the former Yugoslavia included fellatio in the definition of rape, because [para 183]: "The Trial Chamber holds that the forced penetration of the mouth by the male sexual organ constitutes a most humiliating and degrading attack upon human dignity. The essence of the whole corpus of international humanitarian law as well as human rights law lies in the protection of the human dignity of every person, whatever his or her gender."

Countries around the world differ in how they deal with the mens rea element in the law regarding rape, (i.e. the belief of the accused that the victim is not consenting or might not be consenting), and in how they place the onus of proof with regard to belief of consent.

For example, under the Sexual Offences Act 2003, the belief must be "reasonable" and "Whether a belief is reasonable is to be determined having regard to all the circumstances, including any steps A has taken to ascertain whether B consents."