Section 497 of the IPC (Indian Penal Code) perceives a consensual sexual intercourse between a man, married or unmarried, and a married woman without the consent or connivance of her husband as an offence of "adultery". A sexual link between a married or unmarried man and an unmarried woman or a divorcee or a widow, therefore, does not come within the ambit of "adultery". It also holds the man and not the (adulteress) "wife" of another man, who has been unfaithful to her husband, solely responsible for the sexual liaison. IPC, it seems, views "adultery" as an invasion of the right of the husband over his wife and therefore puts it under Chapter XX: "Of Offences Relating to Marriage".
The problem of proving adultery created what is called the disposition and opportunity rule. This rule held a presumption of adultery if it could be shown that the two people in question had an adulterous disposition and had the opportunity to commit adultery.
Even with the rule, proving adultery is difficult. An adulterous disposition usually means showing romantic involvement or entanglement of some sort. Love letters or public (or even semi-public, as we shall see) displays of affection are considered evidence of an adulterous disposition. The opportunity to commit adultery means showing that the two people in question were alone for a time in an apartment, hotel room, or house. Basically, the opportunity to commit adultery means the parties in question had the time and the place to commit adultery. Thus, having the keys to the hotel room but no record of being there was not enough for one spouse to prove adultery against the other one.