In most countries, adultery is a legal ground for divorce (the other being, generally, cruelty).
A person who seduces another's spouse is known as the "adulterer."
In old English law, this gave a husband an action against the adulterer, known as criminal conversation.
Canada's Divorce Act lists adultery as grounds for a divorce (but does not define it):
"A court of competent jurisdiction may, on application by either or both spouses, grant a divorce to the spouse or spouses on the ground that there has been a breakdown of their marriage. Breakdown of a marriage is established only if (separate and apart for at least one year), or the spouse against whom the divorce proceeding is brought has, since celebration of the marriage, committed adultery...."
A consistent legal definition of adultery has bewitched Courts of law for as long as there has been the concept of marriage.
North Carolina's version is worth noting (G.S. 14-184), as it is found under "criminal law":
"Fornication and adultery.... any man and woman, not being married to each other, ... lewdly and lasciviously associate, bed and cohabit together...."
To "lewdly and lasciviously associate". Now those are words to loudly throw out in a Court of law.
Canadian Courts have not been without their own epiphanies on the topic:
"The fact that it has been held that anything short of actual sexual intercourse, no matter how indecent or improper the act may be, does not constitute adultery, really tends to strengthen my view that it is not the moral turpitude that is involved, but the invasion of the reproductive function.
"So long as nothing takes place which can by any possibility affect that function, there can be no adultery; so that, unless and until there is actual sexual intercourse, there can be no adultery. But to argue, from that, that adultery necessarily begins and ends there is utterly fallacious.
"Sexual intercourse is adulterous because in the case of the woman it involves the possibility of introducing into the family of the husband a false strain of blood. Any act on the part of the wife which does that would, therefore, be adulterous."
Orford v Orford 58 DLR 251(1921, Ontario Supreme Court)
Sexual intercourse is therefore required and, in Canada, to date, neither bestiality nor homosexual acts will suffice or qualify.
In Babineau v Babineau (4 DLR 951), a 1924 New Brunswick (Canada) case, a wife tried to get a divorce on the grounds of bestiality of which, the Court said, "there is abundant evidence".
But the Court reluctantly concluded that as grounds for a divorce, adultery does not include bestiality.
The Court took the unusual initiative to conclude its judgment in these words:
"I have with great reluctance come to this conclusion, and would have been very glad if I could possibly have seen any way by which, under the circumstances, I could have found any possible reason, to authorize entering the decree asked for."
A woman raped is not subject to a claim of adultery nor is a husband or wife who, because of mental disability, is unable to understand what they are doing.
Adultery cases from the law books can read like espisodes of Jerry Springer. Forster v Forster was a 1955 Ontario Court of Appeal case, the facts of which were:
"She said that she had deceived her husband with reference to these trips to Toronto, and had told him that she had been visiting a girl friend. She said that she had been in the hotel bedroom with the co-respondent on the night in question for from one and a half to two hours; that she lay on the bed because she was not feeling well due to the presence of one of her menstrual periods and that the co-respondent lay beside her and kissed her. She explained the fact that her brassiere was found on a chair in the room to the circumstance that she had broken a strap of this article in trying to look out of the bedroom window and had then removed the brassiere without taking off her dress, immediately before her husband had entered the room. She said that she and the co-respondent lay on the bed together for an hour and a half. The co-respondent testified that when the respondent had first entered the room he had locked the door, but that he did not lie down on the bed, and had kissed the respondent only twice."
The Court of Appeal believed the defendant wife, her excuse being apparent from this extract:
"The defendant spouse supported her denial by her statement that her menstrual period began the night before the act of adultery is said to have taken place and that it did not take place, the inference from her evidence being that sexual intercourse did not take place because of the fact of her menstrual period. The conduct of the parties bears all the ear-marks of adultery. There was clearly propensity, clandestinity and opportunity and, as the learned trial Judge finds, even willingness on the part of the defendant spouse except for the fact of her menstrual period."
Canada's Criminal Code does not identify adultery as an offence per se except within the context of endangering the morals of a child:
"Every one who, in the home of a child, participates in adultery ... or any other form of vice, and thereby endangers the morals of the child or renders the home an unfit place for the child to be in, is guilty of an indictable offence and liable to imprisonment for a term not exceeding two years."
In Bowman v Bowman,  2 All E.R. 127, Lord Denning wrote:
"The husband who commits adultery within a few weeks of marriage, or who commits adultery promiscuously with more than one woman or with his wife's sister, or with a servant in the house, may probably be labelled as exceptionally depraved."
There is another very real and terrifying perspective on adultery (called "zina" in Islam Law), pointedly set out in this extract of the Penal Code of the northern Nigerian state of Zamfara, Ch. 8, s. 126:
"Whoever, being a man or a woman fully responsible, has sexual intercourse through the genital of a person over whom he has no sexual rights and in circumstances in which no doubt exists as to the illegality of the act, is guilty of the offence of zina.
"Whoever commits the offence of zina shall be punished with caning of one hundred lashes if unmarried, and shall also be liable to imprisonment for a term of one year; or if married, with stoning to death."