If you are on the lead up to leaving and believe your spouse has been behaving unreasonably, you might want to sharpen up on the clearer definition of it, especially if you’re planning on divorcing them in the not too distant future.
You will have to cite it in reasonable detail on your divorce petition.
It is unpleasant or extreme behaviour from your spouse which has led to you feeling you are unable to live with them any more. There isn’t a singular legal definition as such but it certainly includes physical and mental cruelty to you or your children, verbal abuse, financial irresponsibility, drunkenness or transmission of certain sexually transmitted diseases to you.
There will generally have to have been a number of incidents and each incident does not necessarily have to be serious, however a prolonged period of trivial incidents may be considered sufficient to justify a divorce.
Do your best to provide a good account of the cumulative behaviour that you consider has led to the breakdown of your marriage and be sure to include dates where possible.
The end is near, hang on in there.