Rape is forced, non-consensual sexual intercourse, and a form of sexual assault. Rape is, in most societies, a criminal offense, and is widely considered to be immoral.
Under the common law of the United Kingdom, Australia and the United States, rape describes forced sexual intercourse. Most of these countries have since criminalized marital or spousal rape, which common law did not recognize as an offense. In the United Kingdom, this was as a result of R v. R in 1991. India passed the Protection of Women from Domestic Violence Act 2005 which created a civil remedy for marital rape.
In 1993, the United Nations published the Declaration on the Elimination of Violence against Women which established, among other things, marital rape as a human rights violation.
In the United States, there is no national, federal rape law. The individual states have laws against rape - and some only have male-on-female rape laws, with male-on-male or female-perpetrated rapes being prosecuted under other legislation.
In many legal systems, there is a mens rea element to rape laws requiring that the accused be aware that the victim is not consenting or might not be consenting. What level of proof, and who the onus of proof is placed on, differs between jurisdictions. Simply believing that a person consents is often not enough to excuse an act if the consent is not reasonable.
Due to a number of factors including feelings of fear, vulnerability and a feeling that one incited one's own rape, many rapes go unreported to the police. Of the rapes that do get reported, many can't be prosecuted because of lack of evidence.
-  1 AC 599, a House of Lords Judicial Committee case