A sin can be grossly defined as 'something that would hurt others' sentiments/their way of living/their comforts and their solace, since the act of the perpetrator has resulted in the betrayal of the beliefs of the affected'. That way, since adultery goes against promises made and destroys the belief system of the affected, it is a sin.
That definition is not complete.
Because then you saying that sin has be against another individual or human grouping.
What about killing of animals?
Human sentiments are very fragile.
For instance I invest in stock, i buy and sell. My gain is someone else loss. That is the nature of the world. A tiger has to kill a dear to survive.
There is nothing called absolute sin, that is universal.
My definition of sin is our association with certain action and the motive behind the action.
Most other religions or philosophies have lists of things that you shall not do. There seem to be such moral absolutes like gluttony, pride, adultery, lying, robbery, not worshiping 5 times a day etc. This has always seemed as a very rigid system to me. Because of this definition, we are constantly commiting sins. For example, we all seem to use what we call “white lies” to smoothen our daily interactions and tactfully deal with people. In such cases it even seems appropriate and “the right thing to do”.
Gita Chapter 18 verse 47
śreyān sva-dharmo viguṇaḥ
para-dharmāt sv-anuṣṭhitāt
svabhāva-niyataḿ karma
kurvan nāpnoti kilbiṣam
It is better to engage in one’s own occupation, even though one may perform it imperfectly, than to accept another’s occupation and perform it perfectly. Duties prescribed according to one’s nature are never affected by sinful reactions.
Anything done for personal sense gratification is a cause of bondage. The conclusion is that everyone should be engaged according to the particular mode of nature he has acquired, and he should decide to work only to serve the supreme cause of the Supreme Lord.
Papa and punya are purely a stamp of religion on practices of the society at that time and place.
Hinduism unlike other religion does not proscribe a list of actions.