Adultery has been defined under Section 497 of the Indian Penal Code as –
‘Whoever has sexual intercourse with a person who is and whom he knows or has reason to believe to be the wife of another man, without the consent or connivance of that man, such sexual intercourse not amounting to the offence of rape, is guilty of the offence of adultery, and shall be punished with imprisonment of either description for a term which may extend to five years, or with fine, or with both. In such case the wife shall not be punishable as an abettor.’

In simpler terms, whoever has sexual intercourse with the wife of another man, without his knowledge or consent is guilty of the offence of adultery. A major ground for divorce under all marriage laws, adultery gives rise to a number of social complications, exacerbated if a child is born out of the liaison. As is clear from the provision of law above, the definition of adultery does not include adultery by a woman.

However, as per section 125(4) of the Criminal Procedure Code, no wife shall be entitled to receive an allowance of maintenance from her husband under the section if she is living in adultery. 
The burden of proving adultery in a matrimonial case is on the person who makes the allegation