This judgment from Apex court defines what is mental cruelty as envisaged under section 13(1)(ia) of Hindu Marriage Act.

Mental cruelty broadly means, when either party causes mental pain, agony or suffering of such a magnitude that it severs the bond between the wife and husband and as a result of which it becomes impossible for the party who has suffered to live with the other party. In other words, the party who has committed wrong is not expected to live with the other party.